[LLDB][MIPS] Make register read/write to set/get the size of register according to abi.

Summary:
For O32 abi register size should be 4 bytes.
For N32 and N64 abi register size should be 8 bytes.
This patch will make register read/write to set/get the size of register according to abi.

Reviewers: clayborg, tberghammer
Subscribers: lldb-commits, nitesh.jain, mohit.bhakkad, bhushan, jaydeep
Differential: http://reviews.llvm.org/D15884
llvm-svn: 256834