[builtins] ARM: Reland fix for assembling builtins in thumb state.

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.

Also add a fix to ensure that armv7k-watchos can assemble the
aeabi_c{f|d}cmp.S files.

Fixes PR 34715.

Reviewers: compnerd, peter.smith, srhines, weimingz, rengolin, efriedma, t.p.northover, fjricci

Reviewed By: compnerd

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

Differential Revision: https://reviews.llvm.org/D38390

llvm-svn: 314718