efi/libstub: Add $(CLANG_FLAGS) to x86 flags

When cross compiling x86 on an ARM machine with clang, there are several
errors along the lines of:

arch/x86/include/asm/page_64.h:52:7: error: invalid output constraint '=D' in asm

This happens because the x86 flags in the EFI stub are not derived from
KBUILD_CFLAGS like the other architectures are and the clang flags that
set the target architecture ('--target=') and the path to the GNU cross
tools ('--prefix=') are not present, meaning that the host architecture
is targeted.

These flags are available as $(CLANG_FLAGS) from the main Makefile so
add them to the cflags for x86 so that cross compiling works as expected.

Signed-off-by: Nathan Chancellor <[e-mail]>
Signed-off-by: Borislav Petkov <[e-mail]>
Acked-by: Ard Biesheuvel <[e-mail]>
Link: https://lkml.kernel.org/r/[e-mail]

drivers/firmware/efi/libstub/Makefile

···
 				   -Wno-pointer-sign \
 				   $(call cc-disable-warning, address-of-packed-member) \
 				   $(call cc-disable-warning, gnu) \
-				   -fno-asynchronous-unwind-tables
+				   -fno-asynchronous-unwind-tables \
+				   $(CLANG_FLAGS)
 
 # arm64 uses the full KBUILD_CFLAGS so it's necessary to explicitly
 # disable the stackleak plugin