Add condition variable implementation for VxWorks

[jphickey]

Is your feature request related to a problem? Please describe.
Pull Request #1278 added a condition variable API implementation for POSIX, but RTEMS and VxWorks were set to use the "not-impl" module, with a plan to implement in the future.

Describe the solution you'd like
Add an implementation of condition variables for VxWorks 7.x

Additional Context
Only VxWorks 7.x is likely to be supportable, version 6.x will still need to use the "not-impl" module because the underlying sync mechanism may not exist.

Requester Info
Joseph Hickey, Vantage Systems, Inc.