ability to link against darwin frameworks (such as CoreFoundation) when cross compiling

[kristate]

The std library now has code that must be linked against CoreFoundation on macOS and iOS.

What is the best way to do so?

[andrewrk]

Here's what we need to accomplish:

• Ability to link against darwin frameworks when cross-compiling, without any kind of SDK installed.
• Only link against frameworks that are actually used. This is currently accomplished for dynamic libraries with the extern "libname" syntax.