Drop lying to the compiler in the fletcher4 code

This is probably the uncontroversial part of #13631, which fixes
a real problem people are having.

There's still things to improve in our code after this is merged,
but it should stop the breakage that people have reported, where
we lie about a type always being aligned and then pass in stack
objects with no alignment requirement and hope for the best.

Of course, our SIMD code was written with unaligned accesses, so it
doesn't care if we drop this...but some auto-vectorized code that
gcc emits sure does, since we told it it can assume they're aligned.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Tino Reichardt <milky-zfs@mcmilk.de>
Reviewed-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Signed-off-by: Rich Ercolani <rincebrain@gmail.com>
Closes #14649