[mips] Implement .ent, .end, .frame, .mask and .fmask assembler directives

Patch by Matheus Almeida and Toma Tabacu

Differential Revision: http://reviews.llvm.org/D4179


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@215359 91177308-0d34-0410-b5e6-96231b3b80d8