[clang][arm64] Disable -Wasm-operand-widths warning

Clang incorrectly diagnoses msr operations as need a 64-bit operand even
if the underlying register is actually 32 bits. Silence this warning.
There are quite a few occurrences of this warning so I opted to add the
-Wno-flag instead of wrapping all callsites in
`#pragma clang diagnostic ignored -Wasm-operand-widths`.

engine.mk

@@ -197,6 +197,15 @@ STRIP := $(TOOLCHAIN_PREFIX)strip
 ifeq ($(call is_warning_flag_supported,-Wnonnull-compare),yes)
 GLOBAL_COMPILEFLAGS += -Wno-nonnull-compare
 endif
+# Ideally we would move this check to arm64/rules.mk, but we can only check
+# for supported warning flags once CC is defined.
+ifeq ($(ARCH),arm64)
+# Clang incorrectly diagnoses msr operations as need a 64-bit operand even if
+# the underlying register is actually 32 bits. Silence this common warning.
+ifeq ($(call is_warning_flag_supported,-Wasm-operand-widths),yes)
+ARCH_COMPILEFLAGS += -Wno-asm-operand-widths
+endif
+endif
 
 $(info PROJECT = $(PROJECT))
 $(info PLATFORM = $(PLATFORM))