Add and consistently use wrapper macro for romstage static variables

x86 systems run their romstage as execute-in-place from flash, which
prevents them from having writable data segments. In several code pieces
that get linked into both romstage and ramstage, this has been worked
around by using a local variable and having the 'static' storage class
guarded by #ifndef __PRE_RAM__.

However, x86 is the only architecture using execute-in-place (for now),
so it does not make sense to impose the restriction globally. Rather
than fixing the #ifdef at every occurrence, this should really be
wrapped in a way that makes it easier to modify in a single place. The
chromeos/cros_vpd.c file already had a nice approach for a wrapper
macro, but unfortunately restricted it to one file... this patch moves
it to stddef.h and employs it consistently throughout coreboot.

BRANCH=nyan
BUG=None
TEST=Measured boot time on Nyan_Big before and after, confirmed that it
gained 6ms from caching the FMAP in vboot_loader.c.

Original-Change-Id: Ia53b94ab9c6a303b979db7ff20b79e14bc51f9f8
Original-Signed-off-by: Julius Werner <jwerner@chromium.org>
Original-Reviewed-on: https://chromium-review.googlesource.com/203033
Original-Reviewed-by: Aaron Durbin <adurbin@chromium.org>
Original-Reviewed-by: Stefan Reinauer <reinauer@chromium.org>
(cherry picked from commit c8127e4ac9811517f6147cf019ba6a948cdaa4a5)
Signed-off-by: Marc Jones <marc.jones@se-eng.com>

Change-Id: I44dacc10214351992b775aca52d6b776a74ee922
Reviewed-on: http://review.coreboot.org/8055
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>
Tested-by: build bot (Jenkins)

src/include/stddef.h

@@ -49,4 +49,11 @@ typedef unsigned int wint_t;
 #define ROMSTAGE_CONST
 #endif
 
+/* Work around non-writable data segment in execute-in-place romstage on x86. */
+#if defined(__PRE_RAM__) && CONFIG_ARCH_X86
+#define MAYBE_STATIC
+#else
+#define MAYBE_STATIC static
+#endif
+
 #endif /* STDDEF_H */