CHROMIUM: 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. Most of these warnings are fixed in upstream, however we consider it
isn't worth the effort/risk to backport all the necessary changes. On
future CrOS kernels >= v4.13 the warning should be re-enabled.

Change-Id: Ia331bc83d44b8c1499450aefb45c576cd29ebf55
Signed-off-by: Matthias Kaehlcke <mka@chromium.org>
Signed-off-by: Greg Hackmann <ghackmann@google.com>