There are situations where you want to build against a libc that is out
of tree or that is not the system libc (Or for cross build scenarios).
This is a change for passing the -sdk and include paths for things like
this.
The key thing here is that all of the underlying code is exactly the same. I
purposely did not debride anything. This is to ensure that I am not touching too
much and increasing the probability of weird errors from occurring. Thus the
exact same code should be executed... just the routing changed.
Create a new RuntimeUnittest library alongside the other stdlib unit
tests so we can write C++ runtime unit tests callable from lit.
Move runtime exclusivity tests into the stdlib unittest library and
create lit tests so we can verify that the runtime crashes with an
error message.
