[lldb] Only override target arch if it is compatible

It looks like the goal of this code is to provide a more precise
architecture definition for the target when attaching to a process. When
attaching to a foreign debugserver, you might get into a situation where
the active (host) platform will give you bogus information on the target
process.

This change allows the platform to override the target arch only with a
compatible architecture. This fixes TestTargetXMLArch.py on Apple
Silicon. Another alternative would be to just fail in this scenario and
update the test(s).