get rid of std.os.foo.is_the_target

It had the downside of running all the comptime blocks and resolving
all the usingnamespaces of each system, when just trying to discover if
the current system is a particular one.

For Darwin, where it's nice to use `std.Target.current.isDarwin()`, this
demonstrates the utility that #425 would provide.
60cd11bd4bAndrew Kelley committed on 10/24/2019, 5:14:52 AMparent8591731
26 files changedLine totals unavailable