arm64: Disable asm-operand-width warning for clang

clang raises 'asm-operand-widths' warnings in inline assembly code when
the size of an operand is < 64 bits and the operand width is unspecified.
Most warnings are raised in macros, i.e. the datatype of the operand may
vary.
Signed-off-by: Matthias Kaehlcke <mka@chromium.org>

nc: I trimmed the original commit message since I'm not a part of CrOS
    and can't speak on their behalf.

    To fix these warnings, it requires a fairly intrusive backport of
    the sysreg conversion that Mark Rutland did in 4.9. I think
    disabling the warning is smarter, similar to commit d41d0fe3
    ("turn off -Wattribute-alias") in this tree.
Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

arch/arm64/Makefile

@@ -56,6 +56,10 @@ else
 TEXT_OFFSET := 0x00080000
 endif
 
+ifeq ($(cc-name),clang)
+KBUILD_CFLAGS += $(call cc-disable-warning, asm-operand-widths)
+endif
+
 # KASAN_SHADOW_OFFSET = VA_START + (1 << (VA_BITS - 3)) - (1 << 61)
 # in 32-bit arithmetic
 KASAN_SHADOW_OFFSET := $(shell printf "0x%08x00000000\n" $$(( \