Fix broken bootstrap on FreeBSD.

Fix broken bootstrap on FreeBSD.

As analyzed by Steve, on freebsd __gthread_t is a pointer type.
I thought it the cleanest solution to remove the #ifdef in gfc_unit,
make the "self" member a intptr_t and cast the return value of
__gthread_t to that type.

PR fortran/123512

libgfortran/ChangeLog:

	* io/io.h: Change type of self to intptr_t.
	* io/async.h (LOCK_UNIT): Cast __gthread_self () to intptr_t.
	(TRYLOCK_UNIT): Likewise.
	(OWN_THREAD_ID): Likewise.