[libomptarget-nvptx] Ignore calls to dynamic API

There is no support and according to the OpenMP 4.5, p238:7-9:

    For implementations that do not support dynamic adjustment
    of the number of threads this routine has no effect: the
    value of dyn-var remains false.

Add a test that cancellation and nested parallelism aren't
supported either.

Differential Revision: https://reviews.llvm.org/D51785

llvm-svn: 343381