cmd/dist: fix darwin build

The TLS block on Darwin is not the same as on ELF.

TBR=elias.naur
CC=golang-dev
https://golang.org/cl/12741044

src/cmd/dist/buildruntime.c

@@ -166,17 +166,16 @@ static struct {
 	// If the linker is not outputting a shared library, it will reduce
 	// the TLS accessors to the local exec model, effectively removing
 	// get_tls().
-	{"amd64", "",
-		"// The offsets 0 and 8 are known to:\n"
-		"//	../../cmd/6l/pass.c:/D_GS\n"
-		"//	cgo/gcc_linux_amd64.c:/^threadentry\n"
-		"//	cgo/gcc_darwin_amd64.c:/^threadentry\n"
-		"//\n"
+	{"amd64", "linux",
 		"#define	get_tls(r) MOVQ runtime·tlsgm(SB), r\n"
 		"#define	g(r) 0(r)(GS*1)\n"
 		"#define	m(r) 8(r)(GS*1)\n"
 	},
-	
+	{"amd64", "",
+		"#define get_tls(r)\n"
+		"#define g(r) 0(GS)\n"
+		"#define m(r) 8(GS)\n"
+	},	
 	{"arm", "",
 	"#define	LR	R14\n"
 	},

src/cmd/ld/data.c

@@ -1212,7 +1212,7 @@ dodata(void)
 		diag("data or bss segment too large");
 	}
 	
-	if(iself && linkmode == LinkExternal && s != nil && s->type == STLSBSS && HEADTYPE != Hopenbsd) {
+	if(iself && linkmode == LinkExternal && s != nil && s->type == STLSBSS && iself && HEADTYPE != Hopenbsd) {
 		sect = addsection(&segdata, ".tbss", 06);
 		sect->align = PtrSize;
 		sect->vaddr = 0;

src/cmd/ld/lib.c

@@ -360,16 +360,17 @@ loadlib(void)
 				} else
 					s->type = 0;
 			}
-	} 
+	}
+	
 	gmsym = lookup("runtime.tlsgm", 0);
 	gmsym->type = STLSBSS;
 	gmsym->size = 2*PtrSize;
 	gmsym->hide = 1;
-	if(linkmode == LinkExternal)
+	if(linkmode == LinkExternal && iself && HEADTYPE != Hopenbsd)
 		gmsym->reachable = 1;
 	else
 		gmsym->reachable = 0;
-	
+
 	// Now that we know the link mode, trim the dynexp list.
 	x = CgoExportDynamic;
 	if(linkmode == LinkExternal)