Fix warnings about sha2_is_supported on FreeBSD/i386

This is one problem currently preventing OpenZFS from building on
FreeBSD/i386.

Reviewed-by: Alexander Motin <alexander.motin@TrueNAS.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Alan Somers <asomers@gmail.com>
Sponsored by: ConnectWise
Closes #17704