[lldb] [test] Rewrite g/p/G/P tests not to rely on hardcoded ARM regs

Rewrite the register reading/writing tests to use explicit qRegisterInfo
packets rather than relying on ARM registers being hardcoded in LLDB.
While at it, use x86_64 for tests -- since it was easier for me to get
the register lists from that architecture.

Differential Revision: https://reviews.llvm.org/D111496