Log Message:
rust: Set CARGO_BUILD_JOBS to MAKE_JOBS.

cargo defaults to using the number of CPUs detected on the host machine, which
is a terrible heuristic and can easily lead to DRAM exhaustion, especially in a
zones environment where you have access to the full number of CPUs but will be
limited to a portion of available DRAM.

Tidy up a SunOS section while here.