gdb: pass inferior to gdbarch_update_p

Make the current inferior reference bubble up one level.  I think this
makes it clearer what gdbarch_update_p, which is update the passed
inferior's architecture (although the function name could probably be
better).

When gdbarch_find_by_info, it is possible for the new architecture's
init callback to be called.  I have not audited all of them (there are
just too many), it's possible that some of them do care about the
current inferior, for some reason (for instance, if one of them makes a
target call).  If so, they should be changed too.

Change-Id: I89f012188d7fdca395a830f4b013743565f26847