Revert "Disable big-endian simd in swap_nonoverlapping_bytes"
This reverts commit 77bd4dc65406ba3cedbc779e6f6280868231912e (#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
cff1bdbd77