Add target.xml support for qXfer request.

Summary:
Requesting registers one by one takes a while in our project.
We want to get rid of it by using target.xml.

Reviewers: jarin, labath, omjavaid

Reviewed By: labath, omjavaid

Subscribers: omjavaid, lldb-commits

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D74217