Add big-endian support

[zeldin]

This PR adds an option to picorv32 to build it in big endian mode.
The instruction stream format mandated by UCB for big endian systems can be enabled with a separate option. If this option is not enabled, an instruction stream generated for LE can be used (but note that sub-word rodata, such as string literals, will not be correct) in lieu of a big endian tool chain.

Tested on iCE40 HX8K hardware, including compressed instruction streams in the big endian format.

[zeldin]

Note: I have also updated binutils and gcc to support big endian code generation, here and here.

[zeldin]

I updated the code to follow the latest version of the RISC-V specification which calls for fully little-endian instruction encoding on big-endian RISC-V.

Updated toolchains are here:
https://github.com/zeldin/riscv-binutils-gdb/commits/big-endian
https://github.com/zeldin/riscv-gcc/commits/big-endian