AARCH64: Migrate from 'regset_from_core_section' to 'iterate_over_regset_sections'

AARCH64: 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.  This slightly reduces the code and enables non-native
use of the 'generate-core-file' command.

Also, when all instances of 'regset_from_core_section' are replaced,
it can be dropped from the gdbarch interface.

gdb/ChangeLog:

	* aarch64-linux-tdep.c (aarch64_linux_regset_from_core_section):
	Remove.
	(aarch64_linux_iterate_over_regset_sections): New.
	(aarch64_linux_init_abi): Adjust gdbarch initialization.