[GOMP] Fix (un)tied tasks with the GCC

The first bit is actually the "untied" flag. That is why the condition was
wrong and has to be inverted to set the flag correctly.

Found and initial patch by Simon Convent!

git-svn-id: https://llvm.org/svn/llvm-project/openmp/trunk@307899 91177308-0d34-0410-b5e6-96231b3b80d8