Prior to Xcode 9.3, the clang built-in assembler didn't support
altmacro, and gas-preprocessor was used for assembling for arm/darwin.

For thumb functions, gas-preprocessor took care of adding the .thumb_func
directives, but when now being able to assemble without gas-preprocessor,
we need to add these directives ourselves.
Signed-off-by: Martin Storsjö <martin@martin.st>