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 openzfs#16693

include/os/freebsd/spl/sys/simd.h

@@ -50,7 +50,7 @@
 #define	kfpu_fini()		do {} while (0)
 #endif
 
-#define	simd_stat_init()	0
-#define	simd_stat_fini()	0
+#define	simd_stat_init()	do {} while (0)
+#define	simd_stat_fini()	do {} while (0)
 
 #endif