Fix atomic-alignment warnings in libspl on FreeBSD/i386

On i386, Clang complains about misaligned atomic operations. Silence
these warnings to fix the build on FreeBSD/i386.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Alan Somers <asomers@gmail.com>
Sponsored by: ConnectWise
Closes #17708