ALPHA: Migrate from 'regset_from_core_section' to 'iterate_over_regset_sections'

ALPHA: Migrate from 'regset_from_core_section' to 'iterate_over_regset_sections'

Don't define the 'regset_from_core_section' method, but the iterator
method instead.  Do this for GNU/Linux- as well as
Net/OpenBSD-targets.  In the case of GNU/Linux this should enable
non-native use of the 'generate-core-file' command.

gdb/ChangeLog:

	* alpha-linux-tdep.c (alpha_linux_regset_from_core_section): Remove.
	(alpha_linux_iterate_over_regset_sections): New.
	(alpha_linux_init_abi): Adjust gdbarch initialization.
	* alphabsd-tdep.h (alphanbsd_regset_from_core_section): Remove
	prototype.
	(alphanbsd_iterate_over_regset_sections): New prototype.
    	* alphafbsd-tdep.c (alphafbsd_init_abi): Add comment for missing
    	fbsd_init_abi invocation.
	* alphanbsd-tdep.c (alphanbsd_supply_gregset): Move below
	alphanbsd_aout_supply_gregset.  Invoke the latter for the
	appropriate size.
	(alphanbsd_aout_gregset): Remove.
	(alphanbsd_regset_from_core_section): Remove.
	(alphanbsd_iterate_over_regset_sections): New.
	(alphanbsd_init_abi): Adjust gdbarch initialization.
	* alphaobsd-tdep.c (alphaobsd_init_abi): Likewise.