docs: reflect recent changes to `root` and linking

Update `GeneratedIRReference.rst:` to reflect recent changes to the
`root` function. Specifically, the presence of the stack pointer
argument and initialization of the program counter and the stack
pointer.

`FromIRToExecutable.rst` has been updated too, to take into account the
new way we link `support.c`.

Finally, the `--tracing` argument has been removed from
`RevambUsage.rst` and the `-trace` argument is no documented in
`TranslateUsage.rst`.