Drop ROMCC code and header guards

Change-Id: I730f80afd8aad250f26534435aec24bea75a849c
Signed-off-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/37334
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
Reviewed-by: HAOUAS Elyes <ehaouas@noos.fr>

src/include/stddef.h

@@ -47,12 +47,10 @@ typedef unsigned int wint_t;
 #define MAYBE_STATIC_BSS
 #endif
 
-#ifndef __ROMCC__
 /* Provide a pointer to address 0 that thwarts any "accessing this is
  * undefined behaviour and do whatever" trickery in compilers.
  * Use when you _really_ need to read32(zeroptr) (ie. read address 0).
  */
 extern char zeroptr[];
-#endif
 
 #endif /* STDDEF_H */