Provide a gcc-safe zero pointer

zeroptr is a linker object pointing at 0 that can be used to thwart
GCC's (and other compilers') "dereferencing NULL is undefined"
optimization strategy when it gets in the way.

Change-Id: I6aa6f28283281ebae73d6349811e290bf1b99483
Signed-off-by: Patrick Georgi <patrick@georgi-clan.de>
Reviewed-on: https://review.coreboot.org/12294
Tested-by: Raptor Engineering Automated Test Stand <noreply@raptorengineeringinc.com>
Tested-by: build bot (Jenkins)
Reviewed-by: Martin Roth <martinroth@google.com>

src/include/stddef.h

@@ -34,4 +34,12 @@ typedef unsigned int wint_t;
 #define MAYBE_STATIC static
 #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 void *zeroptr;
+#endif
+
 #endif /* STDDEF_H */