[Ada] Tighten crtbegin files for VxWorks

[Ada] Tighten crtbegin files for VxWorks

Enforce a more explicit distinction of crtbegin objects holding
either functions with ctor/dtor attributes or _ctors/_dtors arrays,
or none of the two (no array, no attributes). Then allow/enforce
different linking strategies for VxWorks 7.

2018-05-29  Olivier Hainque  <hainque@adacore.com>

gcc/ada/

	* vx_crtbegin.inc: Use a consistent naming convention for the
	registration/deregistration functions across RTP or kernel.  Remove the
	ctor/dtor attribute setting based on RTP/kernel, expect the optional
	attribute extension to be provided by includers instead.
	* vx_crtbegin.c: Mere inclusion of vx_crtbegin.inc with empty attribute
	extension for the registration/deregistration functions.
	* vx_crtbegin_attr.c: New file. Include vx_crtbegin.inc with explicit
	constructor/destructor attribute extensions.
	* vx_crtbegin_array.c: New file. Include vx_crtbegin.inc with empty
	attribute extensions and declare _ctors/_dtors arrays.
	* vx_crtbegin_auto.c: Remove.
	* libgnat/system-vxworks7-aarch64-rtp-smp.ads: Use
	vxworks7-gnat-crtbe-link.spec.
	* libgnat/system-vxworks7-aarch64.ads: Likewise.
	* libgnat/system-vxworks7-e500-rtp-smp.ads: Likewise.
	* libgnat/system-vxworks7-ppc-rtp-smp.ads: Likewise.
	* libgnat/system-vxworks7-ppc64-rtp-smp.ads: Likewise.
	* libgnat/system-vxworks7-x86-kernel.ads: Likewise.
	* libgnat/system-vxworks7-x86-rtp-smp.ads: Likewise.
	* libgnat/system-vxworks7-x86_64-kernel.ads: Likewise.
	* libgnat/system-vxworks7-x86_64-rtp-smp.ads: Likewise.

From-SVN: r260876