cmd/internal/obj, cmd/link: darwin dynlink support

This makes it possible for cmd/compile, when run with -dynlink on
darwin/amd64, to generate TLS_LE relocations which the linker then
turns into the appropriate PC-relative GOT load.

Change-Id: I1a71da432608bdb108ff66c22de600100209c873
Reviewed-on: https://go-review.googlesource.com/29393Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: David Crawshaw <crawshaw@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/internal/obj/x86/asm6.go

@@ -2769,7 +2769,7 @@ func vaddr(ctxt *obj.Link, p *obj.Prog, a *obj.Addr, r *obj.Reloc) int64 {
 			log.Fatalf("reloc")
 		}
 
-		if !ctxt.Flag_shared || isAndroid {
+		if !ctxt.Flag_shared || isAndroid || ctxt.Headtype == obj.Hdarwin {
 			r.Type = obj.R_TLS_LE
 			r.Siz = 4
 			r.Off = -1 // caller must fill in

src/cmd/link/internal/amd64/asm.go

@@ -425,7 +425,7 @@ func machoreloc1(s *ld.Symbol, r *ld.Reloc, sectoff int64) int {
 
 	rs := r.Xsym
 
-	if rs.Type == obj.SHOSTOBJ || r.Type == obj.R_PCREL {
+	if rs.Type == obj.SHOSTOBJ || r.Type == obj.R_PCREL || r.Type == obj.R_GOTPCREL {
 		if rs.Dynid < 0 {
 			ld.Errorf(s, "reloc %d to non-macho symbol %s type=%d", r.Type, rs.Name, rs.Type)
 			return -1
@@ -456,6 +456,9 @@ func machoreloc1(s *ld.Symbol, r *ld.Reloc, sectoff int64) int {
 	case obj.R_PCREL:
 		v |= 1 << 24 // pc-relative bit
 		v |= ld.MACHO_X86_64_RELOC_SIGNED << 28
+	case obj.R_GOTPCREL:
+		v |= 1 << 24 // pc-relative bit
+		v |= ld.MACHO_X86_64_RELOC_GOT_LOAD << 28
 	}
 
 	switch r.Siz {