cmd/go: for gccgo run the located binary rather than using PATH

Change-Id: I308588db2942e7110a2be22257843931941d92f1
Reviewed-on: https://go-review.googlesource.com/2622Reviewed-by: Peter Collingbourne <pcc@google.com>
Reviewed-by: David Crawshaw <crawshaw@golang.org>

src/cmd/go/build.go

@@ -1910,7 +1910,7 @@ func (gccgoToolchain) linker() string {
 	return gccgoBin
 }
 
-func (gccgoToolchain) gc(b *builder, p *Package, archive, obj string, asmhdr bool, importArgs []string, gofiles []string) (ofile string, output []byte, err error) {
+func (tools gccgoToolchain) gc(b *builder, p *Package, archive, obj string, asmhdr bool, importArgs []string, gofiles []string) (ofile string, output []byte, err error) {
 	out := "_go_.o"
 	ofile = obj + out
 	gcargs := []string{"-g"}
@@ -1921,7 +1921,7 @@ func (gccgoToolchain) gc(b *builder, p *Package, archive, obj string, asmhdr boo
 	if p.localPrefix != "" {
 		gcargs = append(gcargs, "-fgo-relative-import-path="+p.localPrefix)
 	}
-	args := stringList(gccgoName, importArgs, "-c", gcargs, "-o", ofile, buildGccgoflags)
+	args := stringList(tools.compiler(), importArgs, "-c", gcargs, "-o", ofile, buildGccgoflags)
 	for _, f := range gofiles {
 		args = append(args, mkAbs(p.Dir, f))
 	}
@@ -1930,14 +1930,14 @@ func (gccgoToolchain) gc(b *builder, p *Package, archive, obj string, asmhdr boo
 	return ofile, output, err
 }
 
-func (gccgoToolchain) asm(b *builder, p *Package, obj, ofile, sfile string) error {
+func (tools gccgoToolchain) asm(b *builder, p *Package, obj, ofile, sfile string) error {
 	sfile = mkAbs(p.Dir, sfile)
 	defs := []string{"-D", "GOOS_" + goos, "-D", "GOARCH_" + goarch}
 	if pkgpath := gccgoCleanPkgpath(p); pkgpath != "" {
 		defs = append(defs, `-D`, `GOPKGPATH="`+pkgpath+`"`)
 	}
 	defs = append(defs, b.gccArchArgs()...)
-	return b.run(p.Dir, p.ImportPath, nil, gccgoName, "-I", obj, "-o", ofile, defs, sfile)
+	return b.run(p.Dir, p.ImportPath, nil, tools.compiler(), "-I", obj, "-o", ofile, defs, sfile)
 }
 
 func (gccgoToolchain) pkgpath(basedir string, p *Package) string {
@@ -2016,7 +2016,7 @@ func (tools gccgoToolchain) ld(b *builder, p *Package, out string, allactions []
 	if objc {
 		ldflags = append(ldflags, "-lobjc")
 	}
-	return b.run(".", p.ImportPath, nil, gccgoName, "-o", out, ofiles, "-Wl,-(", ldflags, "-Wl,-)", buildGccgoflags)
+	return b.run(".", p.ImportPath, nil, tools.linker(), "-o", out, ofiles, "-Wl,-(", ldflags, "-Wl,-)", buildGccgoflags)
 }
 
 func (gccgoToolchain) cc(b *builder, p *Package, objdir, ofile, cfile string) error {