Add option to revert to dynamic stdlib

[vzsg]

The statically linked Swift stdlib is cool, allowing for smaller images and better startup times, but not all projects are equal. Some might not run due to missing symbol errors, others might fail to build due to the increased memory pressure during build.

To alleviate this, the previous behavior is now available via a configuration variable. If SWIFT_DYNAMIC_STDLIB is defined with a non-empty value, the stdlib and Foundation remain dynamically linked, and copied into the final image, just like in the good old days.

But this time, with backtrace support.