mm: silence -Woverride-init/initializer-overrides

When compiling a kernel with W=1, there are several of those warnings due
to arm64 overriding a field on purpose.  Just disable those warnings for
both GCC and Clang of this file, so it will help dig "gems" hidden in the
W=1 warnings by reducing some noises.

mm/init-mm.c:39:2: warning: initializer overrides prior initialization
of this subobject [-Winitializer-overrides]
        INIT_MM_CONTEXT(init_mm)
        ^~~~~~~~~~~~~~~~~~~~~~~~
./arch/arm64/include/asm/mmu.h:133:9: note: expanded from macro
'INIT_MM_CONTEXT'
        .pgd = init_pg_dir,
               ^~~~~~~~~~~
mm/init-mm.c:30:10: note: previous initialization is here
        .pgd            = swapper_pg_dir,
                          ^~~~~~~~~~~~~~

Note: there is a side project trying to support explicitly allowing
specific initializer overrides in Clang, but there is no guarantee it
will happen or not.

ClangBuiltLinux#639

Link: http://lkml.kernel.org/r/1566920867-27453-1-git-send-email-cai@lca.pw
Signed-off-by: Qian Cai <cai@lca.pw>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/Makefile

@@ -21,6 +21,9 @@ KCOV_INSTRUMENT_memcontrol.o := n
 KCOV_INSTRUMENT_mmzone.o := n
 KCOV_INSTRUMENT_vmstat.o := n
 
+CFLAGS_init-mm.o += $(call cc-disable-warning, override-init)
+CFLAGS_init-mm.o += $(call cc-disable-warning, initializer-overrides)
+
 mmu-y			:= nommu.o
 mmu-$(CONFIG_MMU)	:= highmem.o memory.o mincore.o \
 			   mlock.o mmap.o mmu_gather.o mprotect.o mremap.o \