Cygwin build is broken

I have a series of PRs required to fix cygwin builds of boost. There's one for each of the submodule changes here:

https://github.com/boostorg/boost/compare/master...corngood:boost:cygwin

These are based on the out-of-date downstream patches in cygwin:

https://cygwin.com/cgit/cygwin-packages/boost/tree/

This issue is to track them in one place:

- https://github.com/boostorg/asio/pull/463
- https://github.com/boostorg/context/pull/322
- https://github.com/boostorg/process/pull/533
- https://github.com/boostorg/stacktrace/pull/219
- https://github.com/boostorg/system/pull/137

Start by reviewing the linked comparison and the five submodule pull requests for the Cygwin-specific changes, then compare them with the downstream Cygwin Boost patches. No individual file or test is named in the issue; the work is complete when the affected Boost submodules build successfully on Cygwin and the tracked changes are integrated.

Written by the indexing model from the issue text.