[Ada] change abort signal on aix

[Arnaud Charlet <charlet at adacore dot com>]

Manually tested on ppc-aix
Tested on i686-linux, committed on trunk

On most plaftorms, we use SIGABRT as SIGADAABORT. However, it appears
that on AIX, SIGABRT can't be used with sigwait(2), so we have to use
another signal. We used to use SIGTERM, but this caused problems for cases
where users want to legitimately intercept this signal. This change
uses SIGEMT instead, as this signal has no standard usage.

Also, old versions of AIX did not support properly the SCHED_RR policy, which
is no longer the case, so change value of Time_Slice_Supported to take
advantage of SCHED_RR when possible.

2007-06-06  Thomas Quinot  <quinot@adacore.com>
	    Arnaud Charlet  <charlet@adacore.com>

	* a-intnam-aix.ads: 
	Adjust comment to account for SIGADAABORT change (SIGEMT is now used
	instead of SIGTERM on AIX).

	* s-osinte-aix.ads (Linker_Options): Use -pthread instead of -lpthreads.
	(Time_Slice_Supported): Set to True.
	use SIGEMT instead of SIGTERM as SIGADAABORT.

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