gdb: introduce 'all_non_exited_process_targets' and 'switch_to_target_no_thread'

gdb: introduce 'all_non_exited_process_targets' and 'switch_to_target_no_thread'

Introduce two new convenience functions:

1. all_non_exited_process_targets: returns a collection of all process
stratum targets that have non-exited inferiors on them.  Useful for
iterating targets.

2. switch_to_target_no_thread: switch the context to the first
inferior of the given target, and to no selected thread.

gdb/ChangeLog:
2020-05-14  Tankut Baris Aktemur  <tankut.baris.aktemur@intel.com>

	* process-stratum-target.h: Include <set>.
	(all_non_exited_process_targets, switch_to_target_no_thread): New
	function declarations.
	* process-stratum-target.c (all_non_exited_process_targets)
	(switch_to_target_no_thread): New function implementations.