[OpenMP] Temporarily disable failing runtime tests for clang-12

Following tests were disabled for clang-11 after upgrading to
version 5.0 in D82963:

1. openmp/runtime/test/env/kmp_set_dispatch_buf.c
2. openmp/runtime/test/worksharing/for/kmp_set_dispatch_buf.c

They are also failing for clang-12. Thus this temporary disabling
until they are fixed.

Reviewed By: ABataev

Differential Revision: https://reviews.llvm.org/D84241