Fix inaccuracies in VxWorks LINK_SPEC

Fix inaccuracies in VxWorks LINK_SPEC

-v needs to generate a -V not -v, as most/all other ports do.

The latter causes collect2 to output exec'd collect-ld with same
switches, which in turn causes a configure test which accumulates
linker switches to contain duplicates, leading to a libstdc++ configure
failure in some configurations.

-V is typically used in such contexts to output the available
emulations.

The change also removes reference to %(link_target), long obsolete.

2021-12-07  Doug Rupp  <rupp@adacore.com>

	* config/vxworks.h (LINK_SPEC): Remove %(link_target).
	Change %{v:-v} to %{v:-V}.