We use one bit of the third reserved swift private tls key.
Also move the functionality into a separate static archive that is
always linked dependent on deployment target.
Many build systems that support Swift don't use swiftc to drive the linker. To make things
easier for these build systems, also use autolinking to pull in the needed compatibility
libraries. This is less ideal than letting the driver add it at link time, since individual
compile jobs don't know whether they're building an executable or not. Introduce a
`-disable-autolink-runtime-compatibility` flag, which build systems that do drive the linker
with swiftc can pass to avoid autolinking.
rdar://problem/50057445