Support fully-static linking

Right now, we only support kind-of-static linking: the `swift_library` targets that the binary depends on are statically linked into the final binary, but the Swift runtime libraries are dynamically linked. This is fine (and in fact, desired) on Apple's mobile platforms (iOS, tvOS, watchOS), but on Linux (and probably macOS), we should offer static linking against the runtime as well.

One interesting catch: Linux toolchains do not come with a static version of `libdispatch`, which would seem to mean you can't do a whole lot of interesting stuff in a statically linked environment. So this might reduce the usefulness and urgency of this feature.

Start by examining how swift_library targets are linked into the final binary and how the Swift runtime libraries are selected for Linux and macOS toolchains. Check the availability of static libdispatch on Linux, then determine what toolchain and platform behavior would need to change for runtime-static linking to work. Done means a supported static-linking path is defined without breaking the existing Apple mobile behavior.

Written by the indexing model from the issue text.