Amazon Linux 2 Support for Swift Build

[cmcgee1024]

Amazon Linux 2 is a little more challenging to support than other Linux distributions. In particular, there is a missing implementation of posix_spawn_file_actions_addchdir for the version of glibc available there. There could be other issues with Swift Build support for that platform too, but this is the most immediate problem.

[jakepetroules]

The main thing in Swift Build is our use of Foundation Process (NSTask), which has an implementation of setting the working directory which is NOT thread safe on platforms lacking posix_spawn_file_actions_addchdir (Amazon Linux 2, OpenBSD, QNX, and older Android versions). To solve this we can adopt swift-subprocess, which has a fork/exec fallback path for environments where posix_spawn_file_actions_addchdir is unavailable.

The other one is llbuild's process spawning code, which has a hard dependency on posix_spawn_file_actions_addchdir, and needs to be tracked separately on the llbuild project.

[cmcgee1024]

See swiftlang/swift-llbuild#980 for the llbuild portion.