[Ada] vxworks: auto-registration of foreign threads

[Pierre-Marie de Rodat <derodat at adacore dot com>]

To make Ada tasks and C threads interoperate better, we have added some
functionality to Self. Suppose a C main program (with threads) calls an
Ada procedure and the Ada procedure calls the tasking runtime system.
Eventually, a call will be made to self. Since the call is not coming
from an Ada task, there will be no corresponding ATCB.

What we do in Self is to catch references that do not come from
recognized Ada tasks, and create an ATCB for the calling thread.

The new ATCB will be "detached" from the normal Ada task master
hierarchy, much like the existing implicitly created signal-server
tasks.

Tested on x86_64-pc-linux-gnu, committed on trunk

2017-09-13  Jerome Guitton  <guitton@adacore.com>

	* libgnarl/s-tpopsp__vxworks-tls.adb,
	libgnarl/s-tpopsp__vxworks-rtp.adb, libgnarl/s-tpopsp__vxworks.adb
	(Self): Register thread if task id is null.

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