[OpenMP] Temporarily require X86 target for parallel_for_codegen.cpp test

Since https://reviews.llvm.org/D101849 this test has been failing
on bots that only enable either Arm or AArch64 targets.

See: https://lab.llvm.org/buildbot/#/builders/107/builds/7601

Temporarily requires X86 for this test while the difference is figured out.