Remove 'arch' field from regset structure.

Remove 'arch' field from regset structure.

Removes the 'arch' field from the regset structure, since it
represents the only "dynamic" data in a regset.  It was referenced in
some regset supply- and collect routines, to get access to the gdbarch
associated with the regset.  Naturally, the affected routines always
have access to the regcache to be supplied to or collected from.  Thus
the gdbarch associated with that regcache can be used instead.