[Ada] Misc code clean ups + PR ada/21053

[Arnaud Charlet <charlet at adacore dot com>]

First part of a clean up consisting in removing references to the front-end
zero cost exception mechanism, which is no longer supported with GCC 4.x

Also clean up code concerning VMS condition handler: the signal
argument array to a VMS condition handler always has a PC & PSL
appended. The argument count (first element) must be decremented by 2 before
passing the signal argument array to SYS$PUTMSG otherwise those fields get
wrongly interpreted and either garbage is output or an access violation
results.

Fix trivial PR ada/21053

Finally, modify conditionals to build a run-time for VxWorks 6 RTPs

2005-08-29  Arnaud Charlet  <charlet@adacore.com>
	    Doug Rupp  <rupp@adacore.com>

	* s-stalib.adb: Add missing pragma Warnings (On) to reenable Warnings
	when needed.
	(Inside_Elab_Final_Code): Moved to init.c to avoid having to keep
	this code in the GNAT run-time.

	* fe.h: Replace GCC_ZCX by Back_End_Exceptions.

	PR ada/21053
	* init.c (__gnat_error_handler [many]): Mark "msg" as const
	(__gnat_error_handler [HPUX]): Mark siginfo parameter as unused

	(__gnat_inside_elab_final_code): Moved here from
	Standard_Library and only defined for the compiler.
	__gnat_error_handler [VMS]: Adjust sigargs to account for PC & PSL.
	(__gnat_inum_to_ivec): Do not define this function on VxWorks when
	using RTPs because directly vectored Interrupt routines are not
	supported on this configuration.
	(getpid): Do not redefine this function on VxWorks when using RTPs
	because this primitive is well supported by the RTP libraries.
	(copy_msg): Correct the code that checks for buffer overflow.
	Discovered during code reading.

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