x86/boot: Disable Clang warnings about GNU extensions

The kernel makes use of several GCC extensions, disable Clang warnings
about that in the boot code, as we already do for the rest of the kernel.

This suppresses the following warning when building with clang:

  ./include/linux/cgroup-defs.h:391:16: warning: field 'cgrp' with variable sized type 'struct cgroup' not at the end of a struct or class is a GNU extension [-Wgnu-variable-sized-type-not-at-end]
        struct cgroup cgrp;
Reported-by: NNick Desaulniers <nick.desaulniers@gmail.com>
Signed-off-by: NMatthias Kaehlcke <mka@chromium.org>
Acked-by: NThomas Gleixner <tglx@linutronix.de>
Cc: Douglas Anderson <dianders@chromium.org>
Cc: Guenter Roeck <groeck@chromium.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/20171030194351.122090-1-mka@chromium.orgSigned-off-by: NIngo Molnar <mingo@kernel.org>

arch/x86/boot/compressed/Makefile

@@ -35,6 +35,7 @@ KBUILD_CFLAGS += -mno-mmx -mno-sse
 KBUILD_CFLAGS += $(call cc-option,-ffreestanding)
 KBUILD_CFLAGS += $(call cc-option,-fno-stack-protector)
 KBUILD_CFLAGS += $(call cc-disable-warning, address-of-packed-member)
+KBUILD_CFLAGS += $(call cc-disable-warning, gnu)
 
 KBUILD_AFLAGS  := $(KBUILD_CFLAGS) -D__ASSEMBLY__
 GCOV_PROFILE := n