Add plugin.process.gdb-remote.use-libraries-svr4 option
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
Differential Revision: https://reviews.llvm.org/D64112