cmd/compile: add //go:embed support

This commit contains the compiler support for //go:embed lines.
The go command passes to the compiler an "embed config"
that maps literal patterns like *.txt to the set of files to embed.
The compiler then lays out the content of those files as static data
in the form of an embed.Files or string or []byte in the final object file.

The test for this code is the end-to-end test hooking up the
embed, cmd/compile, and cmd/go changes, in the next CL.

For #41191.

Change-Id: I916e57f8cc65871dc0044c13d3f90c252a3fe1bf
Reviewed-on: https://go-review.googlesource.com/c/go/+/243944
Trust: Russ Cox <rsc@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

src/cmd/internal/obj/link.go

@@ -401,7 +401,7 @@ type LSym struct {
 	P      []byte
 	R      []Reloc
 
-	Extra *interface{} // *FuncInfo if present
+	Extra *interface{} // *FuncInfo or *FileInfo, if present
 
 	Pkg    string
 	PkgIdx int32
@@ -454,6 +454,33 @@ func (s *LSym) Func() *FuncInfo {
 	return f
 }
 
+// A FileInfo contains extra fields for SDATA symbols backed by files.
+// (If LSym.Extra is a *FileInfo, LSym.P == nil.)
+type FileInfo struct {
+	Name string // name of file to read into object file
+	Size int64  // length of file
+}
+
+// NewFileInfo allocates and returns a FileInfo for LSym.
+func (s *LSym) NewFileInfo() *FileInfo {
+	if s.Extra != nil {
+		log.Fatalf("invalid use of LSym - NewFileInfo with Extra of type %T", *s.Extra)
+	}
+	f := new(FileInfo)
+	s.Extra = new(interface{})
+	*s.Extra = f
+	return f
+}
+
+// File returns the *FileInfo associated with s, or else nil.
+func (s *LSym) File() *FileInfo {
+	if s.Extra == nil {
+		return nil
+	}
+	f, _ := (*s.Extra).(*FileInfo)
+	return f
+}
+
 type InlMark struct {
 	// When unwinding from an instruction in an inlined body, mark
 	// where we should unwind to.