SH: Migrate from 'regset_from_core_section' to 'iterate_over_regset_sections'

SH: Migrate from 'regset_from_core_section' to 'iterate_over_regset_sections'

For Super-H targets, no longer define the gdbarch method
'regset_from_core_section', but the iterator method instead.

gdb/ChangeLog:

	* sh-linux-tdep.c (sh_linux_init_abi): Set tdep fields
	'sizeof_gregset' and 'sizeof_fpregset'.
	* sh-tdep.c (sh_regset_from_core_section): Remove.
	(sh_iterate_over_regset_sections): New.
	(sh_gdbarch_init): Adjust gdbarch initialization.
	* sh-tdep.h (struct gdbarch_tdep): New fields sizeof_gregset and
	sizeof_fpregset.
	* shnbsd-tdep.c (shnbsd_init_abi): Set tdep field
	'sizeof_gregset'.