[clang][ARM] Refactor ComputeLLVMTriple code for ARM

This moves code that sets the architecture name
and Float ABI into two new functions in
ToolChains/Arch/ARM.cpp. Greatly simplifying ComputeLLVMTriple.

Some light refactoring in setArchNameInTriple to
move local variables closer to their first use.

Reviewed By: ostannard

Differential Revision: https://reviews.llvm.org/D98253