gcore, target: allow target to prepare/cleanup for/after core file generation

gcore, target: allow target to prepare/cleanup for/after core file generation

Add new target functions to_prepare_to_generate_core and
to_done_generating_core that are called before and after generating a core
file, respectively.

This allows targets to prepare for core file generation and to clean up
afterwards.

gdb/
	* target.h (target_ops) <to_prepare_to_generate_core>
	<to_done_generating_core>: New.
	(target_prepare_to_generate_core, target_done_generating_core): New.
	* target.c (target_prepare_to_generate_core)
	(target_done_generating_core): New.
	* target-delegates.c: Regenerate.
	* gcore.c: (write_gcore_file): Rename to ...
	(write_gcore_file_1): ...this.
	(write_gcore_file): Call target_prepare_to_generate_core
	and target_done_generating_core.