MIPS: Migrate from 'regset_from_core_section' to 'iterate_over_regset_sections'

MIPS: Migrate from 'regset_from_core_section' to 'iterate_over_regset_sections'

For MIPS targets, no longer define the gdbarch method
'regset_from_core_section', but the iterator method instead.

gdb/ChangeLog:

	* mips-linux-tdep.c (mips_linux_regset_from_core_section): Remove.
	(mips_linux_iterate_over_regset_sections): New.
	(mips_linux_init_abi): Adjust gdbarch initialization.
	* mips64obsd-tdep.c (mips64obsd_regset_from_core_section): Remove.
	(mips64obsd_iterate_over_regset_sections): New.
	(mips64obsd_init_abi): Adjust.
	* mipsnbsd-tdep.c (mipsnbsd_regset_from_core_section): Remove.
	(mipsnbsd_iterate_over_regset_sections): New.
	(mipsnbsd_init_abi): Adjust.