stddef.h: fix zeroptr's definition

As Aaron pointed out, the old definition made the compiler emit two
memory accesses, to 0 (for derefencing) and then reading at whatever
address could be read from there.

Change-Id: I5cdd53f5bd2d2397c43f09f3e5fa46be08744b01
Found-by: Aaron Durbin <adurbin@chromium.org>
Signed-off-by: Patrick Georgi <pgeorgi@chromium.org>
Reviewed-on: https://review.coreboot.org/14342
Tested-by: build bot (Jenkins)
Reviewed-by: Aaron Durbin <adurbin@chromium.org>

src/include/stddef.h

@@ -39,7 +39,7 @@ typedef unsigned int wint_t;
  * undefined behaviour and do whatever" trickery in compilers.
  * Use when you _really_ need to read32(zeroptr) (ie. read address 0).
  */
-extern void *zeroptr;
+extern char zeroptr[];
 #endif
 
 #endif /* STDDEF_H */