Introduce an internal API for VxWorks version checks

This changes introduces an internal API for VxWorks version checks
within runtime files, a prerequisite to a few fixes coming up for libstdc++
builds on more recent versions of the OS.

2019-12-30  Olivier Hainque  <hainque@adacore.com>

	gcc/
	* config/vxworks/_vxworks-versions.h: New file.
	* config.gcc (*-*-vxworks*): Add it to extra_headers.

	libgcc/
	* config/gthr-vxworks.h: Use _vxworks-versions.h.
	* config/gthr-vxworks-tls.c: Likewise.

From-SVN: r279783