cmd/go: don't assume cc supports -gno-record-gcc-switches

NetBSD's C compiler appears to support -fdebug-prefix-map but
not -gno-record-gcc-switches. Remove assumption that support
for the former implies the latter.

Change-Id: Iecad9e4f497ea4edc1ce440010e6fe19dc3e0566
Reviewed-on: https://go-review.googlesource.com/19686Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/go/build.go

@@ -2930,12 +2930,15 @@ func (b *builder) ccompilerCmd(envvar, defcmd, objdir string) []string {
 
 	// Tell gcc not to include the work directory in object files.
 	if b.gccSupportsFlag("-fdebug-prefix-map=a=b") {
-		// -gno-record-gcc-switches is supported by all gcc/clang
-		// versions that support -fdebug-prefix-map.
-		a = append(a, "-gno-record-gcc-switches")
 		a = append(a, "-fdebug-prefix-map="+b.work+"=/tmp/go-build")
 	}
 
+	// Tell gcc not to include flags in object files, which defeats the
+	// point of -fdebug-prefix-map above.
+	if b.gccSupportsFlag("-gno-record-gcc-switches") {
+		a = append(a, "-gno-record-gcc-switches")
+	}
+
 	// On OS X, some of the compilers behave as if -fno-common
 	// is always set, and the Mach-O linker in 6l/8l assumes this.
 	// See https://golang.org/issue/3253.