unix: refactor how platforms and optimizations are conveyed

In the new world, build.py accepts --host-platform, --target-triple,
--optimizations, and --dest-archive arguments to all invocations.

Dependency archives are now keyed by their target triple and set
of optimizations. Tool chain archives just by their host platform.

Log filee are also disambiguated by their host platform and
target triple as well.

The arguments to control debug and optimizations have been
converged into a single --optimizations argument that is a set of
possible optimization mechanisms. This allows us to only expose
sane optimization configurations.

Not all builds honor the optimizations configuration options. But
the potential for that is now there.

Our distribution archive names have once again changed. The
optimizations are now a standalone component in the file name
since they act as kind of a universal identifier.

Author: indygreg