FindOpenMP: Restore searching system paths
!1481 (merged) has added NO_DEFAULT_PATH
for FindOpenMP
so that it no longer searches in CMAKE_PREFIX_PATH
, but this also excludes searching in system paths, which causes OpenMP to not be found on RISC-V platforms (see #23469 (closed) for details).
This MR removes NO_DEFAULT_PATH
and uses more granular flags instead. This should continue to prevent searching in CMAKE_PREFIX_PATH
and lastly trying system paths.
Fixes #23469 (closed).