[ARM] Allocatable Global Register Variables for ARM

      Provides support for using r6-r11 as globally scoped
      register variables. This requires a -ffixed-rN flag
      in order to reserve rN against general allocation.

      If for a given GRV declaration the corresponding flag
      is not found, or the the register in question is the
      target's FP, we fail with a diagnostic.

      Differential Revision: https://reviews.llvm.org/D68862