Porting mes to RISC-V 64 bit

[W. J. van der Laan]

Over the weekend I have been working on porting mes to RISC-V. I am aiming for 
RV32IM and RV64IM e.g. `riscv32-unknown-linux-elf` and 
`risc64-unknown-linux-elf`. But 64-bit is my primary aim at the moment because 
it is a more common architecture for Linux RISC-V.

I got as far as having `mes-gcc` run on my SiFive Unleashed board, as well as 
having the scaffold and mes tests pass for gcc (except for 70-extern and 
80-setjmp which seem really impractical to handle for gcc). `mescc` also gets 
to the help message but I haven't adapted it in any way for the architecture 
yet.

The most work was adapting the syscall interface. RISC-V Linux uses 
asm-generic's syscall list which has removed many of the legacy syscalls. I've 
switched to using more modern replacements but only for RV64, other 
architectures should be unaffected.

I expect that the biggest challenge in getting mescc to generate RISC-V 
binaries is the assumption that there is a persistent flags register 
(arch:a?->r and such). RISC-V instead has instructions that compare a register 
against another register or zero and branch / set a value directly. So some 
refactoring outside the architecture-specific module will be needed.

The latest state of my work can be found on the 'riscv64' branch here:
https://github.com/laanwj/guix-mes

A required patch to hex2_linker is on the 'riscv' branch here:
https://github.com/laanwj/guix-mescc-tools

How does the development process work? Should I send patches to this mailing 
list for review when they are ready?

W.