parse CPU features when --target is null
parse CPU features when --target is null

- fixes case where features are not parsed when `-target` is null
- `zig builtin -mcpu native+bogus` should return error/list of valid features

zig-lang/zig,zig-lang/zig,andrewrk/zig,raulgrell/zig,raulgrell/zig,andrewrk/zig,raulgrell/zig,zig-lang/zig,raulgrell/zig,raulgrell/zig