Split MAYBE_STATIC to _BSS and _NONZERO variants

These are required to cover the absensce of .data and
.bss sections in some programs, most notably ARCH_X86
in execute-in-place with cache-as-ram.

Change-Id: I80485ebac94b88c5864a949b17ad1dccdfda6a40
Signed-off-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/35003
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Julius Werner <jwerner@chromium.org>

toolchain.inc

@@ -72,7 +72,7 @@ CFLAGS_ppc64	+=
 # stack use, we use 1.5K as heuristic, assuming that we typically have lots
 # of tiny stack frames and the odd large one.
 #
-# Store larger buffers in BSS, use MAYBE_STATIC to share code with __PRE_RAM__
+# Store larger buffers in BSS, use MAYBE_STATIC_BSS to share code with __PRE_RAM__
 # on x86.
 # Since GCCs detection of dynamic array bounds unfortunately seems to be
 # very basic, you'll sometimes have to use a static upper bound for the