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.