ld: remove duplicate bss definitions

The EXTERN lines in elf.h already define these.
That's not a problem for most C compilers, but
apparently it is for some copies of the OS X linker.

Fixes #2167.

R=golang-dev, iant
CC=golang-dev
https://golang.org/cl/4936044

src/cmd/ld/symtab.c

@@ -34,10 +34,7 @@
 #include	"../ld/lib.h"
 #include	"../ld/elf.h"
 
-char *elfstrdat;
-int elfstrsize;
-int maxelfstr;
-int elftextsh;
+static int maxelfstr;
 
 int
 putelfstr(char *s)