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

Modules/FindOpenMP.cmake

@@ -279,7 +279,9 @@ function(_OPENMP_GET_FLAGS LANG FLAG_MODE OPENMP_FLAG_VAR OPENMP_LIB_NAMES_VAR)
                 DOC "Path to the ${_OPENMP_IMPLICIT_LIB_PLAIN} library for OpenMP"
                 HINTS ${OpenMP_${LANG}_IMPLICIT_LINK_DIRS}
                 CMAKE_FIND_ROOT_PATH_BOTH
-                NO_DEFAULT_PATH
+                NO_PACKAGE_ROOT_PATH
+                NO_CMAKE_PATH
+                NO_CMAKE_ENVIRONMENT_PATH
               )
             endif()
             mark_as_advanced(OpenMP_${_OPENMP_IMPLICIT_LIB_PLAIN}_LIBRARY)