[lldb/lldb-server] Add target.xml support for qXfer request.

Summary:
Synthesize target.xml in lldb-server to avoid a long chain of
qRegisterInfo packets, which can be slow over low-latency links.

Reviewers: jarin, labath

Reviewed By: labath

Subscribers: lldb-commits

Tags: #lldb

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