Duplicated code is bad. At least make it consistent.

The getARMTargetCPU and getLLVMArchSuffixForARM functions exist in both
Toolchain.cpp and Tools.cpp.  This stuff needs a thorough overhaul.  In the
meantime, this patch at least makes them consistent.  One version had been
converted to use StringSwitch, and the other version had new Cortex M-series
processors added.

llvm-svn: 153202