Support big-endian architectures

[yperbasis]

intx has been recently updated to support big-endian architectures, so we might want to do the same in Silkworm.

powerpc64 is a big-endian option in CI (see https://github.com/chfast/intx/pull/257).

For local testing one can build for mips or powerpc and run with qemu.

PR #615 partially addresses the issue.