cmd/go: add -O2 to invocation of gccgo

When using the go tool with gccgo, this changes the default
    compilation to use -O2.  The -gccgoflags option can be used to
    override this default.  I think this change better corresponds to what
    people expect when using the go tool.
    
    Reviewed-on: https://go-review.googlesource.com/c/164378

From-SVN: r269299

gcc/go/gofrontend/MERGE

@@ -1,4 +1,4 @@
-66ac9466852d11e968f8fd2ad6ffc7386cee49e1
+d5e4595784fb744f5fa0e0f90e77de9b5448b793
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

libgo/go/cmd/go/internal/work/gccgo.go

@@ -76,7 +76,7 @@ func (tools gccgoToolchain) gc(b *Builder, a *Action, archive string, importcfg
 		gcargs = append(gcargs, "-fgo-relative-import-path="+p.Internal.LocalPrefix)
 	}
 
-	args := str.StringList(tools.compiler(), "-c", gcargs, "-o", ofile, forcedGccgoflags)
+	args := str.StringList(tools.compiler(), "-c", "-O2", gcargs, "-o", ofile, forcedGccgoflags)
 	if importcfg != nil {
 		if b.gccSupportsFlag(args[:1], "-fgo-importcfg=/dev/null") {
 			if err := b.writeFile(objdir+"importcfg", importcfg); err != nil {