Turn target_can_execute_reverse into function

Turn target_can_execute_reverse into function

This changes target_can_execute_reverse from an object-like macro to
an inline function.

gdb/ChangeLog
2020-09-28  Tom Tromey  <tom@tromey.com>

	* mi/mi-main.c (exec_reverse_continue)
	(mi_cmd_list_target_features): Update.
	* infrun.c (set_exec_direction_func): Update.
	* target.c (default_execution_direction): Update.
	* reverse.c (exec_reverse_once): Update.
	* target.h (target_can_execute_reverse): Now a function.