cmd/go: use -installsuffix to determine the includes directory list

Currently, the directories generaed by includeArgs can have the "_race"
suffix added if invoked with -race flag, but ignores -installsuffix if
set.

R=adg, rsc
CC=golang-dev
https://golang.org/cl/14174043

src/cmd/go/build.go

@@ -1052,8 +1052,8 @@ func (b *builder) includeArgs(flag string, all []*action) []string {
 				dir = filepath.Join(dir, "gccgo_"+goos+"_"+goarch)
 			} else {
 				dir = filepath.Join(dir, goos+"_"+goarch)
-				if buildRace {
-					dir += "_race"
+				if buildContext.InstallSuffix != "" {
+					dir += "_" + buildContext.InstallSuffix
 				}
 			}
 			inc = append(inc, flag, dir)