cmd/compile, cmd/link: add docs

These are the Go 1.4 docs but refreshed for Go 1.5.
The most sigificant change is that all references to the Plan 9 toolchain are gone.
The tools no longer bear any meaningful resemblance.

Change-Id: I44f5cadb832a982323d7fee0b77673e55d761b35
Reviewed-on: https://go-review.googlesource.com/10298
Reviewed-by: Rob Pike <r@golang.org>

src/cmd/compile/doc.go

@@ -0,0 +1,124 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+/*
+Compile, typically invoked as ``go tool compile,'' compiles a single Go package
+comprising the files named on the command line. It then writes a single
+object file named for the basename of the first source file with a .o suffix.
+The object file can then be combined with other objects into a package archive
+or passed directly to the linker (``go tool link''). If invoked with -pack, the compiler
+writes an archive directly, bypassing the intermediate object file.
+
+The generated files contain type information about the symbols exported by
+the package and about types used by symbols imported by the package from
+other packages. It is therefore not necessary when compiling client C of
+package P to read the files of P's dependencies, only the compiled output of P.
+
+Command Line
+
+Usage:
+
+	go tool compile [flags] file...
+
+The specified files must be Go source files and all part of the same package.
+The same compiler is used for all target operating systems and architectures.
+The GOOS and GOARCH environment variables set the desired target.
+
+Flags:
+
+	-D path
+		Set relative path for local imports.
+	-I dir1 -I dir2
+		Search for imported packages in dir1, dir2, etc,
+		after consulting $GOROOT/pkg/$GOOS_$GOARCH.
+	-L
+		Show complete file path in error messages.
+	-N
+		Disable optimizations.
+	-S
+		Print assembly listing to standard output (code only).
+	-S -S
+		Print assembly listing to standard output (code and data).
+	-V
+		Print compiler version and exit.
+	-asmhdr file
+		Write assembly header to file.
+	-complete
+		Assume package has no non-Go components.
+	-cpuprofile file
+		Write a CPU profile for the compilation to file.
+	-dynlink
+		Allow references to Go symbols in shared libraries (experimental).
+	-e
+		Remove the limit on the number of errors reported (default limit is 10).
+	-h
+		Halt with a stack trace at the first error detected.
+	-installsuffix suffix
+		Look for packages in $GOROOT/pkg/$GOOS_$GOARCH_suffix
+		instead of $GOROOT/pkg/$GOOS_$GOARCH.
+	-largemodel
+		Generated code that assumes a large memory model.
+	-memprofile file
+		Write memory profile for the compilation to file.
+	-memprofilerate rate
+		Set runtime.MemProfileRate for the compilation to rate.
+	-nolocalimports
+		Disallow local (relative) imports.
+	-o file
+		Write object to file (default file.o or, with -pack, file.a).
+	-p path
+		Set expected package import path for the code being compiled,
+		and diagnose imports that would cause a circular dependency.
+	-pack
+		Write a package (archive) file rather than an object file
+	-race
+		Compile with race detector enabled.
+	-u
+		Disallow importing packages not marked as safe; implies -nolocalimports.
+
+There are also a number of debugging flags; run the command with no arguments
+for a usage message.
+
+Compiler Directives
+
+The compiler accepts compiler directives in the form of // comments at the
+beginning of a line. To distinguish them from non-directive comments, the directives
+require no space between the slashes and the name of the directive. However, since
+they are comments, tools unaware of the directive convention or of a particular
+directive can skip over a directive like any other comment.
+
+	//line path/to/file:linenumber
+
+The //line directive specifies that the source line that follows should be recorded
+as having come from the given file path and line number. Successive lines are
+recorded using increasing line numbers, until the next directive. This directive
+typically appears in machine-generated code, so that compilers and debuggers
+will show lines in the original input to the generator.
+
+The //line directive is an historical special case; all other directives are of the form
+//go:name, indicating that the directive is defined by the Go toolchain.
+
+	//go:noescape
+
+The //go:noescape directive specifies that the next declaration in the file, which
+must be a func without a body (meaning that it has an implementation not written
+in Go) does not allow any of the pointers passed as arguments to escape into the
+heap or into the values returned from the function. This information can be used as
+during the compiler's escape analysis of Go code calling the function.
+
+	//go:nosplit
+
+The //go:nosplit directive specifies that the next function declared in the file must
+not include a stack overflow check. This is most commonly used by low-level
+runtime sources invoked at times when it is unsafe for the calling goroutine to be
+preempted.
+
+	//go:linkname localname importpath.name
+
+The //go:linkname directive instructs the compiler to use ``importpath.name' as the
+object file symbol name for the variable or function declared as ``localname'' in the
+source code. Because this directive can subvert the type system and package
+modularity, it is only enabled in files that have imported "unsafe".
+*/
+package main