zig build system: add setLinkerScript and setTarget

* See #204 - zig build system
 * allow builtin types Os, Environ, Arch to be used at runtime.
   they have zero_bits=false and debug info now.
 * Eradicate use of `@alloca` in std for syscalls. See #225
 * add `std.io.writeFile`
 * `std.debug.panic` adds a newline to format
c400cb429aAndrew Kelley committed on 4/3/2017, 9:04:46 AMparenta1363b5
10 files changedLine totals unavailable