[compiler-rt] Produce the right arch suffix for arm libraries

If producing libraries with an arch suffix (i.e. if
LLVM_ENABLE_PER_TARGET_RUNTIME_DIR isn't set), we append the
architecture name. However, for arm, clang doesn't look for libraries
with the full architecture name, but only looks for "arm" and "armhf".

Try to deduce what the full target triple might have been, and use
that for deciding between "arm" and "armhf".

This tries to reapply this bit from D98173, that had to be reverted
in 7b153b43 due to affecting how
the builtins themselves are compiled, not only affecting the output
file name.

Differential Revision: https://reviews.llvm.org/D98452