gdb: make gdbarch_register_reggroup_p take a const reggroup *

Change gdbarch_register_reggroup_p to take a 'const struct reggroup *'
argument.  This requires a change to the gdb/gdbarch-components.py
script, regeneration of gdbarch.{c,h}, and then updates to all the
architectures that implement this method.

There should be no user visible changes after this commit.