[LLGS] Limit jThreadsInfo to only the most important registers for now

Summary:
It seems that reading of register data is the biggest bottleneck in LLGS at the moment. Sending
four registers instead of the full GPR set increases the jThreadsInfo processing time about
6-fold. Until we figure out where is this time going, this commit limits the amount of data we
send to provide a more fluid debugging experience.

Reviewers: tberghammer, ovyalov

Subscribers: lldb-commits

Differential Revision: http://reviews.llvm.org/D11264

llvm-svn: 242517