Turn target_have_steppable_watchpoint into function

Turn target_have_steppable_watchpoint into function

This changes the object-like macro target_have_steppable_watchpoint
into an inline function.

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

	* infrun.c (displaced_step_fixup, thread_still_needs_step_over)
	(handle_signal_stop): Update.
	* procfs.c (procfs_target::insert_watchpoint): Update.
	* target.h (target_have_steppable_watchpoint): Now a function.