[OpenMP][omp_lib] Restore compatibility with more restrictive Fortran compilers (#77780)

The most recent changes to `omp_lib.h.var` have re-introduced some
compatibility issues that had to be fixed due to the similar changes in
the past. Namely:

1. D120707 has removed the "use omp_lib_kinds" statement and replaced it
with import

2. D114537 added line continuation to the long lines

This patch introduces the same kind of changes in order to restore
compatibility with some more restrictive Fortran compilers so their
users could still benefit from the LLVM's OpenMP Fortran library.