[flang] Update recently added OpenMP tests to use the new driver

Switching from `%f18` to `%flang_fc1` in LIT tests added in
https://reviews.llvm.org/D91159. This way these tests are run with the
new driver, `flang-new`, when enabled (i.e. when
`FLANG_BUILD_NEW_DRIVER` is set).

Differential Revision: https://reviews.llvm.org/D101078