cmd/cc: emit gc bitmaps in read-only memory

Cuts hello world by 70kB, because we don't write
those names into the symbol table.

Update #6853

LGTM=khr
R=khr
CC=golang-codereviews
https://golang.org/cl/80370045

src/cmd/cc/pgen.c

@@ -51,6 +51,7 @@ makefuncdatasym(char *namefmt, int64 funcdatakind)
 	nod.sym = sym;
 	nod.class = CSTATIC;
 	gins(AFUNCDATA, nodconst(funcdatakind), &nod);
+	linksym(sym)->type = SRODATA;
 	return sym;
 }
 

src/cmd/ld/symtab.c

@@ -425,7 +425,7 @@ symtab(void)
 			s->hide = 1;
 			s->outer = symgofunc;
 		}
-		if(strstr(s->name, ".gcargs·") != nil || strstr(s->name, ".gclocals·") != nil || strncmp(s->name, "gcargs·", 8) == 0 || strncmp(s->name, "gclocals·", 10) == 0) {
+		if(strncmp(s->name, "gcargs.", 7) == 0 || strncmp(s->name, "gclocals.", 9) == 0 || strncmp(s->name, "gclocals·", 10) == 0) {
 			s->type = SGOFUNC;
 			s->hide = 1;
 			s->outer = symgofunc;