x86/build: Add -Wnoaddress-of-packed-member to REALMODE_CFLAGS, to silence GCC9 build warning

BugLink: https://bugs.launchpad.net/bugs/1845405

commit 42e0e954 upstream.

One of the very few warnings I have in the current build comes from
arch/x86/boot/edd.c, where I get the following with a gcc9 build:

   arch/x86/boot/edd.c: In function ‘query_edd’:
   arch/x86/boot/edd.c:148:11: warning: taking address of packed member of ‘struct boot_params’ may result in an unaligned pointer value [-Waddress-of-packed-member]
     148 |  mbrptr = boot_params.edd_mbr_sig_buffer;
         |           ^~~~~~~~~~~

This warning triggers because we throw away all the CFLAGS and then make
a new set for REALMODE_CFLAGS, so the -Wno-address-of-packed-member we
added in the following commit is not present:

  6f303d60 ("gcc-9: silence 'address-of-packed-member' warning")

The simplest solution for now is to adjust the warning for this version
of CFLAGS as well, but it would definitely make sense to examine whether
REALMODE_CFLAGS could be derived from CFLAGS, so that it picks up changes
in the compiler flags environment automatically.
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Acked-by: Borislav Petkov <bp@alien8.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Connor Kuehl <connor.kuehl@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>

arch/x86/Makefile

@@ -38,6 +38,7 @@ REALMODE_CFLAGS	:= $(M16_CFLAGS) -g -Os -D__KERNEL__ \
 
 REALMODE_CFLAGS += $(call __cc-option, $(CC), $(REALMODE_CFLAGS), -ffreestanding)
 REALMODE_CFLAGS += $(call __cc-option, $(CC), $(REALMODE_CFLAGS), -fno-stack-protector)
+REALMODE_CFLAGS += $(call __cc-option, $(CC), $(REALMODE_CFLAGS), -Wno-address-of-packed-member)
 REALMODE_CFLAGS += $(call __cc-option, $(CC), $(REALMODE_CFLAGS), $(cc_stack_align4))
 export REALMODE_CFLAGS