cmd/go: correct gccgo buildid file on ARM

The GNU assembler for ARM treats @ as a comment character, so section
types must be written using % instead.

Fixes https://gcc.gnu.org/PR87260.

Change-Id: I5461e4bf5b20793db321f540c7f25a9e6e12b6f4
Reviewed-on: https://go-review.googlesource.com/135297
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>

src/cmd/go/internal/work/buildid.go

@@ -348,8 +348,12 @@ func (b *Builder) gccgoBuildIDELFFile(a *Action) (string, error) {
 	}
 	fmt.Fprintf(&buf, "\n")
 	if cfg.Goos != "solaris" {
-		fmt.Fprintf(&buf, "\t"+`.section .note.GNU-stack,"",@progbits`+"\n")
-		fmt.Fprintf(&buf, "\t"+`.section .note.GNU-split-stack,"",@progbits`+"\n")
+		secType := "@progbits"
+		if cfg.Goarch == "arm" {
+			secType = "%progbits"
+		}
+		fmt.Fprintf(&buf, "\t"+`.section .note.GNU-stack,"",%s`+"\n", secType)
+		fmt.Fprintf(&buf, "\t"+`.section .note.GNU-split-stack,"",%s`+"\n", secType)
 	}
 
 	if cfg.BuildN || cfg.BuildX {