Add plugin.process.gdb-remote.use-libraries-svr4 option

Summary:
This option allow the toggling of the libraries-svr4 usage in ProcessGDBRemote. It's a follow up of https://reviews.llvm.org/D62503#1564296 and it's meant to test / tweak this new packet with, hopefully, minimum impact and in a faster way.

Enable it with `settings set plugin.process.gdb-remote.use-libraries-svr4 true`. For now, by default it's false.

I didn't put tests up for this but I did test it manually.

Reviewers: labath, jankratochvil

Reviewed By: labath

Subscribers: lldb-commits

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D64112

llvm-svn: 365059