Add SIMD metadata in /proc on Linux follow up

This change accidentally broke the FreeBSD build due to
a conflict between the simd_stat_init()/simd_stat_fini()
macros on FreeBSD and the extern function prototype.

Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #16552