Have GetSupportedArchitectures report all supported arches

PlatformDarwinKernel::GetSupportedArchitectures returns a list
of architectures that are possible for this platform; it was using
a compile-time check for the debug host to decide the list of arches
that were valid.  This was copied from a codepath doing native process
debugging, and was clearly wrong for kernel debugging, but it had
not happened to cause problems so it went unnoticed for a long time.

Small NFC change to the logging messages of Target::SetArchitecture
to make them a little more explicit about how the architecture is
being modified/replaced.

Differential Revision: https://reviews.llvm.org/D137301
rdar://101690111