[mips] Add support for Virtualization ASE

This includes

  Instructions: tlbginv, tlbginvf, tlbgp, tlbgr, tlbgwi, tlbgwr, hypcall
                mfgc0, mtgc0, mfhgc0, mthgc0, dmfgc0, dmtgc0,

  Assembler directives: .set virt, .set novirt, .module virt, .module novirt

  Attribute: virt

  .MIPS.abiflags: VZ (0x100)

Patch by Vladimir Stefanovic.

Differential Revision: https://reviews.llvm.org/D44905

llvm-svn: 331024