MFC r313941:

Make sure the thread constructor and destructor eventhandlers are
called for all threads belonging to a procedure. Currently the first
thread in a procedure is kept around as an optimisation step and is
never freed. Because the first thread in a procedure is never freed
nor allocated, its destructor and constructor callbacks are never
called which means per thread structures allocated by dtrace and the
Linux emulation layers for example, might be present for threads which
don't need these structures.

This patch adds a thread construction and destruction call for the
first thread in a procedure.

Tested: dtrace, linux emulation
Reviewed by: kib @
Sponsored by: Mellanox Technologies