[LLDB][MIPS] Fix ReadRegisterValue for registers with constant 32 bit size…

Description

[LLDB][MIPS] Fix ReadRegisterValue for registers with constant 32 bit size regardless of ABI

Reviewers: clayborg, tberghammer.
Subscribers: jaydeep, bhushan, sagar, nitesh.jain, lldb-commits.
Differential Revision: http://reviews.llvm.org/D16060