[compiler-rt] Normalize i?86 to i386 and armv* to arm for COMPILER_RT_DEFAULT_TARGET_ARCH

This corresponds to getArchNameForCompilerRTLib in clang; any
32 bit x86 architecture triple (except on android, but those
exceptions are already handled in compiler-rt on a different level)
get the compiler rt library names with i386; arm targets get either
"arm" or "armhf". (Mapping to "armhf" is handled in the toplevel
CMakeLists.txt.)

Differential Revision: https://reviews.llvm.org/D98173