[Ada] Lock-free implementation of protected objects (PR ada/54125)

[Arnaud Charlet <charlet at adacore dot com>]

This patch removes dependency on Support_Atomic_Primitives in the run-time
package System.Atomic_Primitives by introducing a new attribute
Atomic_Always_Lock_Free. This changes allow us to remove all the
Support_Atomic_Primitives defaulted values in the corresponding system
packages and fixes build failures on platform that didn't have
the flag Support_Atomic_Primitives defined in the target system.ads
(PR ada/54125)

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

2012-08-06  Vincent Pucci  <pucci@adacore.com>

PR ada/54125
	* exp_attr.adb (Expand_N_Attribute_Reference): Expand new
	Atomic_Always_Lock_Free attribute.
	* sem_attr.adb (Analyze_Attribute): Analyze new
	Atomic_Always_Lock_Free attribute.
	(Eval_Attribute): Nothing to do with new Atomic_Always_Lock_Free
	attribute.
	* sem_ch9.adb (Allows_Lock_Free_Implementation):
	Support_Atomic_Primitives check replaces previous erroneous
	size check.
	* sem_util.adb, sem_util.ads (Support_Atomic_Primitives):
	New routine.
	* snames.ads-tmpl: New name Name_Atomic_Always_Lock_Free and
	new attribute Attribute_Atomic_Always_Lock_Free defined.
	* s-atopri.adb: Support_Atomic_Primitives checks replaced by
	Atomic_Always_Lock_Free queries.
	* system-aix64.ads, system-aix.ads, system-darwin-ppc.ads,
	system-hpux.ads, system-linux-alpha.ads, system-linux-hppa.ads,
	system-linux-ppc.ads, system-linux-s390.ads,
	system-linux-s390x.ads, system-linux-sh4.ads,
	system-linux-sparc.ads, system-lynxos178-ppc.ads,
	system-lynxos-ppc.ads, system-mingw.ads,
	system-vxworks-arm.ads, system-solaris-sparc.ads,
	system-solaris-sparcv9.ads, system-vms_64.ads,
	system-vxworks-m68k.ads, system-vxworks-mips.ads,
	system-vxworks-ppc.ads, system-vxworks-sparcv9.ads: Flag
	Support_Atomic_Primitives removed.

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