Implementation of build.print_step_rusage.

Implementation of build.print_step_rusage.

On non-unix platforms, does not try to call `getrusage` (and does not attempt to
implement its own shim; that could be follow-on work, though its probably best
to not invest too much effort there, versus using separate dedicated tooling).

On unix platforms, calls libc::rusage and attempts to emit the subset of fields
that are supported on Linux and Mac OS X. Omits groups of related stats which
appear to be unsupported on the platform (due to them all remaining zero).

Adjusts output to compensate for Mac using bytes instead of kb (a well known
discrepancy on Mac OS X). However, so far I observe a lot of strange values
(orders of magnitude wrong) reported on Mac OS X in some cases, so I would not
trust this in that context currently.