Make `pixie.mcext` adapt to the target, write tests, fix `va_list` use.

`pixie.mcext` contains wrappers around standard `libc` functions, it currently has no explicit test infrastructure, doesn't adapt to a LLVM `triple` when it should (OS constants can be different on Linux and BSD systems) and the type of `va_list` is also LLVM `triple`/target dependent. This all needs fixing.