Enable cross-compilation with ninja.
Replaced the specialness of _host with _target, b/c the _host
toolchain is usually the vanilla gcc/g++, so it looks better to
leave those variables alone (plus, it mirrors make.py and
Added an AR to the family b/c it differs between host & target.
Made generator_supports_multiple_toolsets depend on the presence
of any of the $FOO_target variables as an intermediate hack
around the double-building problem.