sys-libs/libomp: Perform PDS checks only for relevant kernel versions

sys-libs/libomp: Perform PDS checks only for relevant kernel versions

Update the PDS check logic to apply only when running the Linux kernel,
versions between 4.13 and 4.15. That covers the range of -pf kernels
that have the broken PDS version, and I think we can reasonably assume
users will not be updating the patch along with the kernel.

Also, perform the check only once in pkg_pretend. There is really
no point in repeating it as packages do not alter kernel configuration.

Closes: https://bugs.gentoo.org/638410