cmd/go: fix TestCgoConsistentResults when using clang instead of gcc

As Ian said at:
https://github.com/golang/go/issues/19964#issuecomment-296347750

> the -fdebug-prefix-map option is being applied to the debug info but
> not to the initial .file pseudo-op.
>
> My only current thought for how to fix this is that instead of
> compiling $WORK/a/b/foo.c, we should change the command to (cd
> $WORK/a/b && clang -g -c foo.c). We'll still want
> -fdebug-prefix-map, I think, but that should fix the .file
> pseudo-op.

This CL does that.

Fixes #19964

Change-Id: I442b1201cab9e0448fc520ab243ad364d59cd7c3
Reviewed-on: https://go-review.googlesource.com/41629
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

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

@@ -2977,7 +2977,10 @@ func (b *Builder) gfortran(p *load.Package, out string, flags []string, ffile st
 func (b *Builder) ccompile(p *load.Package, outfile string, flags []string, file string, compiler []string) error {
 	file = mkAbs(p.Dir, file)
 	desc := p.ImportPath
-	output, err := b.runOut(p.Dir, desc, nil, compiler, flags, "-o", outfile, "-c", file)
+	if !filepath.IsAbs(outfile) {
+		outfile = filepath.Join(p.Dir, outfile)
+	}
+	output, err := b.runOut(filepath.Dir(file), desc, nil, compiler, flags, "-o", outfile, "-c", filepath.Base(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.