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Introduce LINUX_VERSION_STR, LINUX_VERSION_CODE macro for use instead

Description

Introduce LINUX_VERSION_STR, LINUX_VERSION_CODE macro for use instead
of harcoded pr_osrelease, pr_osrel values. This will be used later in
the VDSO.

Differential Revision: https://reviews.freebsd.org/D1042
Reviewed by: trasz