Fix the armv7 thumb builtins on darwin

The arm builtins converted into thumb in r213481 are not working
on darwin. On apple platforms, .thumb_func directive is required
to generated correct symbols for thumb functions.

<rdar://problem/18523605>

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@219040 91177308-0d34-0410-b5e6-96231b3b80d8