Increase use of svr4 packets to improve performance on POSIX remotes

Summary:
Allows the remote to enumerate the link map when adding and removing
shared libraries, so that lldb doesn't need to read it manually from
the remote's memory.

This provides very large speedups (on the order of 50%) in total
startup time when using the ds2 remote on android or Tizen devices.

Reviewers: ADodds, tberghammer, tfiala

Subscribers: tberghammer, sas, danalbert, llvm-commits, srhines

Differential Revision: http://reviews.llvm.org/D16004

Change by Francis Ricci <fjricci@fb.com>

llvm-svn: 257502