crossbuild: Make the CHECK_TIME variable work on Linux

Linux /usr/bin/find doesn't understand the -mtime -0s flag.
Instead create a temporary file and compare that file's mtime to
sys/sys/param.h to check whether the clock is correct.

Reviewed By: jhb, imp
Approved By: jhb (mentor)
Differential Revision: https://reviews.freebsd.org/D14157