Select a fletcher 4 implementation.
.sp
Supported selectors are: \fBfastest\fR, \fBscalar\fR, \fBsse2\fR, \fBssse3\fR,
-\fBavx2\fR, and \fBavx512f\fR.
+\fBavx2\fR, \fBavx512f\fR, and \fBaarch64_neon\fR.
All of the selectors except \fBfastest\fR and \fBscalar\fR require instruction
set extensions to be available and will only appear if ZFS detects that they are
present at runtime. If multiple implementations of fletcher 4 are available,