Fix usage of inferior_ptid in two thread_alive implementations

Fix usage of inferior_ptid in two thread_alive implementations

While inspecting some target code, I noticed that in these two
implementations of thread_alive, inferior_ptid is referenced directly
instead of using the ptid passed as parameters.  I guess that it is
wrong, although I can't really test it in both cases.

gdb/ChangeLog:

	* bsd-uthread.c (bsd_uthread_thread_alive): Use ptid instead of
	inferior_ptid.
	* go32-nat.c (go32_thread_alive): Likewise.