[OpenMP] Fix doacross testing for gcc

This patch adds a test using the doacross clauses in OpenMP and removes gcc from
testing kmp_doacross_check.c which is only testing the kmp rather than the
gomp interface.

Differential Revision: https://reviews.llvm.org/D50014


git-svn-id: https://llvm.org/svn/llvm-project/openmp/trunk@338757 91177308-0d34-0410-b5e6-96231b3b80d8