[AsmParser][ARM] Make .thumb_func imply .thumb

GNU as documentation states that a `.thumb_func` directive implies `.thumb`, teach the asm parser to switch mode whenever it's encountered. On the other hand the labeled form, exclusive to Apple's toolchain, doesn't switch mode at all.

Reviewed By: nickdesaulniers, peter.smith

Differential Revision: https://reviews.llvm.org/D101975