Speed up NativeProcessLinux::GetLoadedModuleFileSpec

Summary:
GetLoadedModuleFileSpec was reading /proc/pid/maps character by character, which was very slow,
since we do that for every shared library, which android tends to have a lot. Switching to
ProcFileReader saves us about 0.4 seconds in attach time.

Reviewers: tberghammer

Subscribers: tberghammer, danalbert, lldb-commits

Differential Revision: http://reviews.llvm.org/D11460

llvm-svn: 243019