[compiler-rt] Fix invalid triple on ARM build

The fuzzer build was failing on armv7l, with an invalid triple
error. This happened because CMake's get_compiler_rt_target
function was missing some code to correctly handle arm archs,
such as armhf.

This was originaly part of https://reviews.llvm.org/D140011, that
landed on main with commit cd173cbd.

Fixes #60115

Differential Revision: https://reviews.llvm.org/D142906