[compiler-rt] support armv4t

The main thing that needed changing was excluding functionality that
isn't supported on armv4t. So excluding Arm specific builtin assembly files.

In the process some files were renamed and the source was annotated where
appropriate, so it's a bit easier to follow what group of files are meant for
what purpose.

Reviewed By: peter.smith, nickdesaulniers

Differential Revision: https://reviews.llvm.org/D138725