displaced step: pass down target_waitstatus instead of gdb_signal

displaced step: pass down target_waitstatus instead of gdb_signal

This commit tweaks displaced_step_finish & friends to pass down a
target_waitstatus instead of a gdb_signal.  This needed because a
patch later in the series will want to make
displaced_step_buffers::finish handle TARGET_WAITKIND_THREAD_EXITED.
It also helps with the TARGET_WAITKIND_THREAD_CLONED patch.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=27338
Change-Id: I4c5d338647b028071bc498c4e47063795a2db4c0