Allow target overrides of ctors/dtors support advertisement on VxWorks

Allow target overrides of ctors/dtors support advertisement on VxWorks

2018-11-16  Jerome Lambourg  <lambourg@adacore.com>

        * config/vxworks.h (TARGET_VXWORKS_HAVE_CTORS_DTORS): New macro.
        Default to TARGET_VXWORKS_RTP.
        (SUPPORTS_INIT_PRIORITY): Use TARGET_VXWORKS_HAVE_CTORS_DTORS instead
        of TARGET_VXWORKS_RTP.
        * config/vxworksae.h: Also define TARGET_VXWORKS_HAVE_CTORS_DTORS.
        * config/vxworks.c: Use TARGET_VXWORKS_HAVE_CTORS_DTORS instead
        of TARGET_VXWORKS_RTP to set targetm.have_ctors_dtors.

From-SVN: r266201