This change was mostly made with `zig fmt` and this also modified some whitespace. Note that in some files, `zig fmt` produced incorrect code, so the change was made manually.
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.