FindOpenMP: Restore searching system paths In commit 98314d536e (FindOpenMP: Use NO_DEFAULT_PATH where appropriate, 2017-11-15, v3.11.0-rc1~334^2) we added `NO_DEFAULT_PATH` so that it no longer searches in `CMAKE_PREFIX_PATH`, but this also excludes searching in system paths, which are needed on RISC-V platforms. Use more granular exclusions instead. Fixes: #23469