6l, 8l: elfsetstring is only for .shstrtab

Was also recording for .dynstrtab which made the
table run out of space and would have caused confusion
if the ELF code tried to refer to any of the strings.

R=r
CC=golang-dev
https://golang.org/cl/2288041

src/cmd/6l/asm.c

@@ -138,6 +138,7 @@ addstring(Sym *s, char *str)
 	s->reachable = 1;
 	r = s->size;
 	n = strlen(str)+1;
+	if(strcmp(s->name, ".shstrtab") == 0)
 		elfsetstring(str, r);
 	while(n > 0) {
 		m = n;

src/cmd/8l/asm.c

@@ -129,6 +129,7 @@ addstring(Sym *s, char *str)
 	s->reachable = 1;
 	r = s->size;
 	n = strlen(str)+1;
+	if(strcmp(s->name, ".shstrtab") == 0)
 		elfsetstring(str, r);
 	while(n > 0) {
 		m = n;