[ARM][CMSE] Add commandline option and feature macro

[ARM][CMSE] Add commandline option and feature macro

Defines macro ARM_FEATURE_CMSE to 1 for v8-M targets and introduces
-mcmse option which for v8-M targets sets ARM_FEATURE_CMSE to 3.
A diagnostic is produced when the option is given on architectures
without support for Security Extensions.
Reviewed By: dmgreen, snidertm
Differential Revision: https://reviews.llvm.org/D59879

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@361261 91177308-0d34-0410-b5e6-96231b3b80d8