[Builtins] ARM: Fix assembling files in thumb mode.

Summary:
clang does not assemble files in thumb mode unless .thumb declaration
is present. Add .thumb/.arm decl to _FUNCTION macros to ensure that
files are assembled correctly.

Fixes PR 34715.

Reviewers: compnerd, peter.smith, srhines, weimingz, rengolin

Reviewed By: compnerd

Subscribers: aemerson, javed.absar, llvm-commits, kristof.beyls

Differential Revision: https://reviews.llvm.org/D38227

llvm-svn: 314285