Introduce CPUStringIsValid() into MCSubtargetInfo and use it for ARM .cpu parsing.

Previously .cpu directive in ARM assembler didnt switch to the new CPU and
therefore acted as a nop. This implemented real action for .cpu and eg. 
allows to assembler FreeBSD kernel with -integrated-as.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@223147 91177308-0d34-0410-b5e6-96231b3b80d8