xcompile: Disable null pointer optimizations

According to the C standard, accessing the NULL pointer (memory at
address zero) is undefined behaviour, and so GCC is allowed to optimize
it out. Of course, accessing this memory location is sometimes
necessary, so this optimization can be disabled using
-fno-delete-null-pointer-checks. This is already done in coreboot, but
adding it to xcompile will also disable it for all the payloads. For
example, coreinfo compiled with LTO libpayload crashes when this flag
isn't set, presumably because the compiler is optimizing something out
that it shouldn't.

Change-Id: I4492277f02418ade3fe7a75304e8e0611f49ef36
Signed-off-by: Jacob Garber <jgarber1@ualberta.ca>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/38289
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

Makefile.inc

@@ -414,7 +414,6 @@ CFLAGS_common += -Wstrict-aliasing -Wshadow -Wdate-time -Wtype-limits -Wvla
 CFLAGS_common += -fno-common -ffreestanding -fno-builtin -fomit-frame-pointer
 CFLAGS_common += -ffunction-sections -fdata-sections -fno-pie
 ifeq ($(CONFIG_COMPILER_GCC),y)
-CFLAGS_common += -fno-delete-null-pointer-checks
 # Don't add these GCC specific flags when running scan-build
 ifeq ($(CCC_ANALYZER_OUTPUT_FORMAT),)
 CFLAGS_common += -Wno-packed-not-aligned

util/xcompile/xcompile

@@ -221,7 +221,7 @@ SUBARCH_SUPPORTED+=${TSUPP-${TARCH}}
 GCC_CC_${TARCH}:=${GCC}
 GCC_CFLAGS_${TARCH}:=${CFLAGS_GCC}
 # Generally available for GCC's cc1:
-GCC_CFLAGS_${TARCH}+=-Wlogical-op
+GCC_CFLAGS_${TARCH}+=-fno-delete-null-pointer-checks -Wlogical-op
 GCC_ADAFLAGS_${TARCH}:=${CFLAGS_GCC}
 GCC_COMPILER_RT_${TARCH}:=${CC_RT_GCC}
 GCC_COMPILER_RT_FLAGS_${TARCH}:=${CC_RT_EXTRA_GCC}