When building the new aegis128 NEON code in big endian mode, Clang
complains about the const uint8x16_t permute vectors in the following
way:

  crypto/aegis128-neon-inner.c:58:40: warning: vector initializers are not
      compatible with NEON intrinsics in big endian mode
      [-Wnonportable-vector-initialization]
                static const uint8x16_t shift_rows = {
                                                     ^
  crypto/aegis128-neon-inner.c:58:40: note: consider using vld1q_u8() to
      initialize a vector from memory, or vcombine_u8(vcreate_u8(), vcreate_u8())
      to initialize from integer constants

Since the same issue applies to the uint8x16x4_t loads of the AES Sbox,
update those references as well. However, since GCC does not implement
the vld1q_u8_x4() intrinsic, switch from IS_ENABLED() to a preprocessor
conditional to conditionally include this code.
Reported-by: NNathan Chancellor <natechancellor@gmail.com>
Signed-off-by: NArd Biesheuvel <ard.biesheuvel@linaro.org>
Tested-by: NNathan Chancellor <natechancellor@gmail.com>
Signed-off-by: NHerbert Xu <herbert@gondor.apana.org.au>