Disable big-endian simd in swap_nonoverlapping_bytes

Disable big-endian simd in swap_nonoverlapping_bytes

This is a workaround for #42778, which was git-bisected to #40454's
optimizations to `mem::swap`, later moved to `ptr` in #42819.  Natively
compiled rustc couldn't even compile stage1 libcore on powerpc64 and
s390x, but they work fine without this `repr(simd)`.  Since powerpc64le
works OK, it seems probably related to being big-endian.

The underlying problem is not yet known, but this at least makes those
architectures functional again in the meantime.

cc @arielb1