cmd/ld: fix -s flag for ELF executables

This fixes a regression introduced in changeset 98034d036d03
which added support for producing host object files.

R=rsc, minux.ma
CC=dave, golang-dev
https://golang.org/cl/7307107

src/cmd/ld/elf.c

@@ -1398,8 +1398,10 @@ elfobj:
 	eh->shstrndx = sh->shnum;
 
 	// put these sections early in the list
-	elfshname(".symtab");
-	elfshname(".strtab");
+	if(!debug['s']) {
+		elfshname(".symtab");
+		elfshname(".strtab");
+	}
 
 	for(sect=segtext.sect; sect!=nil; sect=sect->next)
 		elfshbits(sect);