Fix build on mips

Setting and getting register values as bytes instead of depending on the 128 bit integer support in register value.
This patch will fix the build failure in the release branch.

Reviewers: tberghammer, clayborg, hans
Subscribers: bhushan, nitesh.jain, jaydeep, lldb-commits
Differential: http://reviews.llvm.org/D12275
llvm-svn: 245927