Auto merge of #60588 - cuviper:be-simd-swap, r=alexcrichton

Revert "Disable big-endian simd in swap_nonoverlapping_bytes"

This reverts commit 77bd4dc (#43159).

Issue #42778 was formerly easy to reproduce on two big-endian targets,
`powerpc64` and `s390x`, so we disabled SIMD on this function for all
big-endian targets as a workaround.

I have re-tested this code on `powerpc64` and `s390x`, each with the
bundled LLVM 8 and with external LLVM 7 and LLVM 6, and the problems no
longer appear. So it seems safe to remove this workaround, although I'm
still a little uncomfortable that we never found a root-cause...

Closes #42778.
r? @arielb1

src/libcore/ptr.rs

@@ -374,10 +374,7 @@ unsafe fn swap_nonoverlapping_bytes(x: *mut u8, y: *mut u8, len: usize) {
  // #[repr(simd)], even if we don't actually use this struct directly.
  //
  // FIXME repr(simd) broken on emscripten and redox
- // It's also broken on big-endian powerpc64 and s390x. #42778
- #[cfg_attr(not(any(target_os = "emscripten", target_os = "redox",
- target_endian = "big")),
- repr(simd))]
+ #[cfg_attr(not(any(target_os = "emscripten", target_os = "redox")), repr(simd))]
  struct Block(u64, u64, u64, u64);
  struct UnalignedBlock(u64, u64, u64, u64);