ld: fix Mach-O bss bug

Fixes #1559.

R=rsc
CC=golang-dev, peterGo, rog
https://golang.org/cl/4356046

src/cmd/ld/data.c

@@ -749,7 +749,7 @@ dodata(void)
 	}
 
 	for(s = datap; s != nil; s = s->next) {
-		if(s->np > 0 && s->type == SBSS)	// TODO: necessary?
+		if(s->np > 0 && s->type == SBSS && s->file != nil)	// TODO: necessary?
 			s->type = SDATA;
 		if(s->np > s->size)
 			diag("%s: initialize bounds (%lld < %d)",

src/cmd/ld/ldmacho.c

@@ -581,7 +581,10 @@ ldmacho(Biobuf *f, char *pkg, int64 len, char *pn)
 			else
 				s->type = SRODATA;
 		} else {
-			s->type = SDATA;
+			if (strcmp(sect->name, "__bss") == 0)
+				s->type = SBSS;
+			else
+				s->type = SDATA;
 		}
 		if(s->type == STEXT) {
 			if(etextp)