Fix gcc unused value warning in FreeBSD simd.h

The macros simd_stat_init() and simd_stat_fini() in FreeBSD's
simd.h are defined as zero, but they are actually only used as
statements. Replace the definitions with do {} while (0) instead, to
avoid gcc -Wunused-value warnings.

Reviewed-by: Tino Reichardt <milky-zfs@mcmilk.de>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Toomas Soome <tsoome@me.com>
Signed-off-by: Dimitry Andric <dimitry@andric.com>
Closes #16693