The runtime and stubs are built for ALL targets, not specific ones. This allows
us to configure when cross-compiling to Windows again. Collapse the dual
addition of the swiftRuntime into a single build. This unifies the runtime
build for the apple and non-Apple SDKs. The difference here was the ObjC
interop sources. In order to deal with that unification add a CPP macro to
indicate whether the interop sources should be included or not.
To minimize code size and VM live set, we try to funnel all one-time initialization through swift_once instead of mixing it with the C++ runtime's support for lazy static initialization.
...and explicitly mark symbols we export, either for use by executables or for runtime-stdlib interaction. Until the stdlib supports resilience we have to allow programs to link to these SPI symbols.
Correct format:
```
//===--- Name of file - Description ----------------------------*- Lang -*-===//
```
Notes:
* Comment line should be exactly 80 chars.
* Padding: Pad with dashes after "Description" to reach 80 chars.
* "Name of file", "Description" and "Lang" are all optional.
* In case of missing "Lang": drop the "-*-" markers.
* In case of missing space: drop one, two or three dashes before "Name of file".
