RFC: riscv64 big endian system attempt

[Ben Dooks]

With the latest spec adding configurable endianness, we thought we
should try putting together a proof of concept riscv64 big endian.

The full information is documented on our gitlab[1] which includes
source repositories, build information and project documentation.

We have a minimal buildroot, qemu and kernel working on QEMU.

As this is a work in progress any review or help is appreciated.

[1] https://gitlab.com/CodethinkLabs/riscv_bigendian