Revert "Disable big-endian simd in swap_nonoverlapping_bytes"

This reverts commit 77bd4dc654.

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...

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);