[lldb] [gdb-remote] Support client fallback for servers without reg defs

Provide minimal register definition defaults for working with servers
that implement neither target.xml nor qRegisterInfo packets.  This is
useful e.g. when interacting with FreeBSD's kernel minimal gdbserver
that does not send target.xml but uses the same layout for its supported
register subset as GDB.

The prerequisite for this is the ability to determine the correct
architecture, e.g. from the target executable.

Differential Revision: https://reviews.llvm.org/D116896