config: cache results of kernel checks (#17106)

Kernel checks are the heaviest part of the configure checks. This allows
the results to be cached through the normal autoconf cache.

Since we don't want to reuse cached values for different kernels, but
don't want to discard the entire cache on every kernel, we instead add a
short checksum to kernel config cache keys, based on the version and
path, so the cache can hold results for multiple different kernels.

Sponsored-by: https://despairlabs.com/sponsor/

Signed-off-by: Rob Norris <robn@despairlabs.com>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>