In most cases instead pass `for_machine`, the name of the relevant
machines (what compilers target, what targets run on, etc). This allows
us to use the cross code path in the native case, deduplicating the
code.
As one can see, environment got bigger as more information is kept
structured there, while ninjabackend got a smaller. Overall a few amount
of lines were added, but the hope is what's added is a lot simpler than
what's removed.
Also adds test case for shared library linking.
Closes#4912, at least on Linux. The future 0.50.0 does not yet claim to
support CUDA on systems other than Linux and backends other than Ninja.
Dylan Baker