Fix -mthumb resetting of target cpu

Passing -mthumb with no explicit CPU on the command line
resulted in target CPU changing from the architecture
default to arm7tdmi. Now it does not.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@197151 91177308-0d34-0410-b5e6-96231b3b80d8
Make thumb the default instruction set for v7m and v7em

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@197152 91177308-0d34-0410-b5e6-96231b3b80d8
Refactor duplicate functions

getARMCPU and getLLVMArchSuffixForARM existed as very similar functions
in both ToolChain.cpp and Tools.cpp. Create a single implementation of
each in Tools.cpp, eliminate the duplicate and share via Tools.h.

Creates an 'arm' namespace in Tools.h to be used by any ARM-targetting tools.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@197153 91177308-0d34-0410-b5e6-96231b3b80d8
Simple refactoring to eliminate single-use local variable

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@197154 91177308-0d34-0410-b5e6-96231b3b80d8