Do not report -Wasm-operand-widths for ARM output operands. <rdar://14050337>

We're getting reports of this warning getting triggered in cases where it
is not adding any value. There is no asm operand modifier that you can use
to silence it, and there's really nothing wrong with having an LDRB, for
example, with a "char" output.

llvm-svn: 183172