cmd/link: elide individual gcbits symbols

Same as we do for string symbols.

Fixes #11583.

Change-Id: Ia9264f6faf486697d987051b7f9851d37d8ad381
Reviewed-on: https://go-review.googlesource.com/12531Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/internal/obj/link.go

@@ -337,6 +337,7 @@ const (
 	SSTRING
 	SGOSTRING
 	SGOFUNC
+	SGCBITS
 	SRODATA
 	SFUNCTAB
 	STYPELINK

src/cmd/link/internal/ld/dwarf.go

@@ -1419,6 +1419,9 @@ func defdwsymb(sym *LSym, s string, t int, v int64, size int64, ver int, gotype
 	if strings.HasPrefix(s, "go.string.") {
 		return
 	}
+	if strings.HasPrefix(s, "runtime.gcbits.") {
+		return
+	}
 
 	if strings.HasPrefix(s, "type.") && s != "type.*" && !strings.HasPrefix(s, "type..") {
 		defgotype(sym)

src/cmd/link/internal/ld/lib.go

@@ -1793,6 +1793,7 @@ func genasmsym(put func(*LSym, string, int, int64, int64, int, *LSym)) {
 			obj.SSTRING,
 			obj.SGOSTRING,
 			obj.SGOFUNC,
+			obj.SGCBITS,
 			obj.SWINDOWS:
 			if !s.Reachable {
 				continue

src/cmd/link/internal/ld/symtab.go

@@ -373,6 +373,13 @@ func symtab() {
 	s.Reachable = true
 	symgofunc := s
 
+	s = Linklookup(Ctxt, "runtime.gcbits.*", 0)
+	s.Type = obj.SGCBITS
+	s.Local = true
+	s.Size = 0
+	s.Reachable = true
+	symgcbits := s
+
 	symtypelink := Linklookup(Ctxt, "runtime.typelink", 0)
 
 	symt = Linklookup(Ctxt, "runtime.symtab", 0)
@@ -411,6 +418,12 @@ func symtab() {
 			s.Outer = symgostring
 		}
 
+		if strings.HasPrefix(s.Name, "runtime.gcbits.") {
+			s.Type = obj.SGCBITS
+			s.Hide = 1
+			s.Outer = symgcbits
+		}
+
 		if strings.HasPrefix(s.Name, "go.func.") {
 			s.Type = obj.SGOFUNC
 			s.Hide = 1

src/cmd/newlink/testdata/Makefile

@@ -9,7 +9,7 @@ ALL=\
 all: $(ALL)
 
 %.6: %.s
-	GOARCH=amd64 GOOS=darwin go tool asm -I $(shell go env GOROOT)/pkg/include -trimpath=$(shell pwd) $*.s
+	GOARCH=amd64 GOOS=darwin go tool asm -o $*.6 -I $(shell go env GOROOT)/pkg/include -trimpath=$(shell pwd) $*.s
 
 pclntab.s: genpcln.go
 	go run genpcln.go >pclntab.s