go/build: support compiler as build constraint

This supports writing different .c/.s code when using gccgo.

R=golang-dev, dsymonds, iant, rsc
CC=golang-dev
https://golang.org/cl/6823055

src/pkg/go/build/build.go

@@ -897,6 +897,8 @@ func splitQuoted(s string) (r []string, err error) {
 //	$GOARCH
 //	cgo (if cgo is enabled)
 //	!cgo (if cgo is disabled)
+//	ctxt.Compiler
+//	!ctxt.Compiler
 //	tag (if tag is listed in ctxt.BuildTags)
 //	!tag (if tag is not listed in ctxt.BuildTags)
 //	a comma-separated list of any of these
@@ -928,7 +930,7 @@ func (ctxt *Context) match(name string) bool {
 	if ctxt.CgoEnabled && name == "cgo" {
 		return true
 	}
-	if name == ctxt.GOOS || name == ctxt.GOARCH {
+	if name == ctxt.GOOS || name == ctxt.GOARCH || name == ctxt.Compiler {
 		return true
 	}
 

src/pkg/go/build/doc.go

@@ -88,6 +88,7 @@
 //
 //	- the target operating system, as spelled by runtime.GOOS
 //	- the target architecture, as spelled by runtime.GOARCH
+//	- the compiler being used, currently either "gc" or "gccgo"
 //	- "cgo", if ctxt.CgoEnabled is true
 //	- any additional words listed in ctxt.BuildTags
 //