cmd/go: if we get a C compiler dwarf2 warning, try without -g

This avoids a problem that occurs on FreeBSD 11, in which the clang
3.8 assembler issues a pointless warning when invoked with -g on a
file that contains an empty .note.GNU-stack section.

No test because there is no reasonable way to write one, but should
fix the build on FreeBSD 11.

Fixes #14705.

Change-Id: I8c49bbf79a2c715c0e75495da19045fc92280e81
Reviewed-on: https://go-review.googlesource.com/38072
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

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

@@ -2897,6 +2897,26 @@ func (b *Builder) ccompile(p *load.Package, outfile string, flags []string, file
 	desc := p.ImportPath
 	output, err := b.runOut(p.Dir, desc, nil, compiler, flags, "-o", outfile, "-c", file)
 	if len(output) > 0 {
+		// On FreeBSD 11, when we pass -g to clang 3.8 it
+		// invokes its internal assembler with -dwarf-version=2.
+		// When it sees .section .note.GNU-stack, it warns
+		// "DWARF2 only supports one section per compilation unit".
+		// This warning makes no sense, since the section is empty,
+		// but it confuses people.
+		// We work around the problem by detecting the warning
+		// and dropping -g and trying again.
+		if bytes.Contains(output, []byte("DWARF2 only supports one section per compilation unit")) {
+			newFlags := make([]string, 0, len(flags))
+			for _, f := range flags {
+				if !strings.HasPrefix(f, "-g") {
+					newFlags = append(newFlags, f)
+				}
+			}
+			if len(newFlags) < len(flags) {
+				return b.ccompile(p, outfile, newFlags, file, compiler)
+			}
+		}
+
 		b.showOutput(p.Dir, desc, b.processOutput(output))
 		if err != nil {
 			err = errPrintedOutput