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

[stuartarchibald]

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.