[mips] Add MipsOptionRecord abstraction and use it to implement .reginfo/.MIPS.options

This abstraction allows us to support the various records that can be placed in
the .MIPS.options section in the future. We currently use it to record register
usage information (the ODK_REGINFO record in our ELF64 spec).

Each .MIPS.options record should subclass MipsOptionRecord and provide an
implementation of EmitMipsOptionRecord.

Patch by Matheus Almeida and Toma Tabacu



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@213522 91177308-0d34-0410-b5e6-96231b3b80d8