Amazon Linux 2 support for llbuild

[cmcgee1024]

There are certain downstream repositories, like Swift Build, that will be supporting Amazon Linux 2. There needs to be AL2 support in llbuild in order to achieve this.

One particular problem that has been encountered is an message like this following:

working-directory unsupported on this platform

This message occurs when the platform doesn't have a posix_spawn_file_actions_addchdir. To support AL2 there will need to be a work around for this limitation since processes can be spawned in other working directories from the parent.

[jakepetroules]

This will also unblock the OpenBSD port of Swift.