litex_setup: Add installation support for OpenRisc/LM32/PowerPC GCC toolchains.

[enjoy-digital]

RISC-V toolchain can be currently installed with ./litex_setup.py gcc and provides an easy way for users to install the RISC-V GCC toolchain while also being convenient for CI regression tests of the CPUs.

We should also provide installation support for the other supported CPUs.

[mithro]

Just FYI, there are a bunch of packaging efforts - like cross compilers needed for soft-CPUs used in like LiteX under the https://github.com/hdl organization. This includes distribution with things like conda, containers, native packages and static binaries.

In theory https://github.com/hdl/conda-compilers was suppose to provide cross-compilers for all supported LiteX architectures on Linux + Mac + Windows. I'm unclear what the status is despite it being tracked at https://j.mp/edda-status -- @PiotrZierhoffer - Do you know where things got too?

@umarcor Do you know the state of cross compiler toolchains for the various architectures LiteX needs in the other distribution systems?

[umarcor]

On MSYS2, I know several "popular" architectures are supported (arm-none-eabi, avr, m68k-apple-macos, powerpc-apple-macos, riscv64-unknown-elf). I do use the riscv64 to compile software for NEORV32. However, I don't maintain those packages on MINGW-packages. That's done by other people in the MSYS2 community.

On hdl/containers, we don't provide software compilation tooling yet. I am focused on HDL's, and FPGAs, slowly introducing ASIC tooling. Providing images for other HDLs/languages/frameworks such as LiteX, Chisel or SpinalHDL falls within my scope (see hdl/containers#16), but building software toolchains is not something I can add to my list. Nevertheless, contributions and shared maintenance would be welcome in that area.