Deprecate riscv gcc

[SebastianBoe]

There are two changes in this pull-request.

Change the way we satisfy our compiler-dependency from being git
submodule based to being "Go there and compile it from source you
twat" based. And change what compiler we use.

Optimally our compiler-dependency would be satisified by a
package, but no-one has created one yet and I found it
disappointingly tricky to create one myself. I believe that it is
unnecessary to have a git submodule when we are not going to
contribute back to the compiler. But this can be debated, thus
the pull-request.

We change the compiler used from riscv-gcc to riscv-gnu-toolchain
because riscv-gcc is deprecated[1].

[1] riscvarchive/riscv-old-gcc#17 (comment)