Hoist shape discrimination above thin lock owner test in the lock

Hoist shape discrimination above thin lock owner test in the lock
procedure.  It is unsafe to reckon a thin lock owner without having
first determined that the bit pattern of the lock word corresponds to
that of a thin lock.  Without proper ordering, a monitor lock can and
will be created which, excluding the shape bit, corresponds to a lock
owned by the calling thread.

In addition, move compiler barriers so they immediately preceed base
address publication.  Also, kill cargo-cult volatiles that confounded
my bug hunt.