SwiftAsync: use runtime-provided flag for extended frame if back-deploying

When back-deploying Swift async code we can't always toggle the flag showing an
extended frame is present because it will confuse unwinders on systems released
before this feature. So in cases where the code might run there, we `or` in a
mask provided by the runtime (as an absolute symbol) telling us whether the
unwinders can cope.

When deploying only for newer OSs, we can still hard-code the bit-set for
greater efficiency.