Add check_member macro to allow clean and easy struct offset checking

This patch adds a new static assertion macro that can be used to check
the offsets in structures that overlay register sets at compile time. It
uses the _Static_assert() declaration from the new ISO C11 standard,
which is supported (even without -std=c11) by GCC after version 4.6.
(There is supposedly also support in clang, although I haven't tried
it... let's deal with compiler issues when/if they turn up.)

I've added it to all structures for our current ARM SoCs for now, and I
think every new register overlay we add going forward should use them
(at least for the last member, but feel free to add more if you think
it's useful).

Change-Id: If32510e7049739ad05618d363a854dc372d64386
Signed-off-by: Julius Werner <jwerner@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/179412
Reviewed-by: David Hendricks <dhendrix@chromium.org>
Reviewed-by: Stefan Reinauer <reinauer@chromium.org>
(cherry picked from commit cef5fa13c3)
Signed-off-by: Isaac Christensen <isaac.christensen@se-eng.com>
Reviewed-on: http://review.coreboot.org/6905
Tested-by: build bot (Jenkins)

src/include/stddef.h

@@ -28,6 +28,10 @@ typedef unsigned int wint_t;
 
 #define offsetof(TYPE, MEMBER) ((size_t) &((TYPE *)0)->MEMBER)
 
+#define check_member(structure, member, offset) _Static_assert( \
+	offsetof(struct structure, member) == offset, \
+	"`struct " #structure "` offset for `" #member "` is not " #offset )
+
 #ifdef __PRE_RAM__
 #define ROMSTAGE_CONST const
 #else