The cooperative global executor uses <chrono> for its handling of
times. This creates a dependency on a C++11 standard library, and
doesn't actually work with freestanding implementations like we want
for Embedded Swift.
Replace this <chrono> usage with swift_sleep / swift_get_time from
the concurrency library, which already considers various
platform-specific implementations before falling back to <chrono>.
Enable -ffreestanding for the embedded Concurrency library and its
support libraries.
Doug Gregor
64f98f8b57