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
1 file changed