Switch to an ifunc in the kernel for crc32c

There is no need to read the same variable to check if the CPU supports
crc32c instructions.

Reviewed by: arichardson, kib, markj
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D31274

(cherry picked from commit a93941b439fce7047dffad6bc380cc9454b967cd)