FROMLIST: efi/libstub: Disable some warnings for x86{,_64}

When building the kernel with Clang, some disabled warnings appear
because this Makefile overrides KBUILD_CFLAGS for x86{,_64}. Add them to
this list so that the build is clean again.

-Wpointer-sign was disabled for the whole kernel before the beginning
of git history.

-Waddress-of-packed-member was disabled for the whole kernel in
commit bfb38988 ("kbuild: clang: Disable 'address-of-packed-member'
warning") and for x86/boot/compressed in commit 20c6c189 ("x86/boot:
Disable the address-of-packed-member compiler warning").

-Wgnu was disabled for the whole kernel in commit 61163efa ("kbuild:
LLVMLinux: Add Kbuild support for building kernel with Clang") and for
x86/boot/compressed in commit 6c3b56b1 ("x86/boot: Disable Clang
warnings about GNU extensions").

Link: https://github.com/ClangBuiltLinux/linux/issues/112


Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>
Tested-by: Nick Desaulniers <ndesaulniers@google.com>
(am from https://lore.kernel.org/patchwork/patch/999058/)
(also found at https://lkml.kernel.org/r/20181013010348.6740-1-natechancellor@gmail.com

)

BUG=chromium:897984
TEST=build for lakitu

Change-Id: Ibe18a22243f5c99c3ceb5b93f8a648ffaa1c7f66
Signed-off-by: Guenter Roeck <groeck@chromium.org>