NFC refactorings in lib/Support/TargetParser.cpp

Summary:
* declare FPUNames, ARCHNames, ARCHExtNames, HWDivNames, CPUNames
  as static const
* implement getDefaultExtensions with a StringSwitch, in the same
  way getDefaultFPU is implemented

Reviewers: rengolin

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D14648

llvm-svn: 253201