cmd/compile: provide a way to auto-discover -d debug keys

Currently one needs to refer to the sources to have a list of accepted
debug keys. We can copy what 'ssa/help' does and introspect the list of
debug keys to print a more detailed help:

    $ go tool compile -d help
    usage: -d arg[,arg]* and arg is <key>[=<value>]

    <key> is one of:

    	append    	print information about append compilation
    	closure   	print information about closure compilation
    	disablenil	disable nil checks
    	dclstack  	run internal dclstack check
    	gcprog    	print dump of GC programs
    	nil       	print information about nil checks
    	panic     	do not hide any compiler panic
    	slice     	print information about slice compilation
    	typeassert	print information about type assertion inlining
    	wb        	print information about write barriers
    	export    	print export data
    	pctab     	print named pc-value table
    	ssa/help  	print help about SSA debugging

    <value> is key-specific.

    Key "pctab" supports values:
    	"pctospadj", "pctofile", "pctoline", "pctoinline", "pctopcdata"

For '-d help' to be discoverable, a hint is given in the -d flag
description.

A last thing, today at least one go file needs to be provided to get to
the code printing ssa/help.

  $ go tool compile -d ssa/help foo.go

Add a check so one can just do '-d help' or '-d ssa/help'

Caught by trybot: I needed to update fmt_test.go as I'm introducing the
usage of %-*s in a format string.

Fixes #20041

Change-Id: Ib2858b038c1bcbe644aa3b1a371009710c6d957d
Reviewed-on: https://go-review.googlesource.com/41091
Run-TryBot: Alberto Donizetti <alb.donizetti@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>

src/cmd/compile/fmt_test.go

@@ -687,6 +687,7 @@ var knownFormats = map[string]string{
 	"reflect.Type %s":  "",
 	"rune %#U":         "",
 	"rune %c":          "",
+	"string %-*s":      "",
 	"string %-16s":     "",
 	"string %.*s":      "",
 	"string %q":        "",