Inhibit support for gthread condvars on VxWorks < 6

Inhibit support for gthread condvars on VxWorks < 6

The condition variables support relies on kernel entry points
to enforce critical aspects of it's expected behavior.

Some of these entry points are not available prior to VxWorks 6,
so we just expose absence of support for condition variables on
such systems.

2020-10-15  Rasmus Villemoes  <rv@rasmusvillemoes.dk>

libgcc/
	* config/gthr-vxworks.h: Condition the __GTHREAD_HAS_COND
	section on VxWorks >= 6.
	* config/gthr-vxworks-cond.c: Condition the entire
	implementation on __GTHREAD_HAS_COND.