Bump up MAX_SUBTARGET_FEATURES

Summary:
Currently, ARMGenSubtargetInfo (from ARM.td) is reaching the limit of 96:
  enum : uint64_t {
       ...
       XScale = 95
  };
       
We need to bump the maximum value up to accommodate future changes and/or customized subtarget definitions. 

Reviewers: apazos, t.p.northover

Subscribers: llvm-commits, aemerson

Differential Revision: http://reviews.llvm.org/D15514

llvm-svn: 255616