cmd/go: don't generate DWARF in the compiler for plan9

Follow-up to CL 40859.

Change-Id: I785b8b28f179a8f2fcd42b0a97ffcd41de4b21bc
Reviewed-on: https://go-review.googlesource.com/40865
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/go/internal/work/build.go

@@ -2204,7 +2204,7 @@ func (gcToolchain) gc(b *Builder, p *load.Package, archive, obj string, asmhdr b
 		gcargs = append(gcargs, "-buildid", p.Internal.BuildID)
 	}
 	platform := cfg.Goos + "/" + cfg.Goarch
-	if p.Internal.OmitDebug || platform == "nacl/amd64p32" || platform == "darwin/arm" || platform == "darwin/arm64" {
+	if p.Internal.OmitDebug || platform == "nacl/amd64p32" || platform == "darwin/arm" || platform == "darwin/arm64" || cfg.Goos == "plan9" {
 		gcargs = append(gcargs, "-dwarf=false")
 	}