Re-commit: [mips] Implement .ent, .end, .frame, .mask and .fmask.

Patch by Matheus Almeida and Toma Tabacu

The lld test failure on the previous attempt to commit was caused by the
addition of the .pdr section causing the offsets it was checking to change.
This has been fixed by removing the .ent/.end directives from that test since
they weren't really needed.

llvm-svn: 215535