Add support for OpenBSD

[grynspan]

Add support for OpenBSD:

• Audit our platform-specific code and add branches for OpenBSD (mostly, but not entirely, the same as FreeBSD)
• Set up team workstations or VMs
• Set up CI jobs

[grynspan]

As far as I know, the FreeBSD code should all work verbatim except for CommandLine.executablePath.

[grynspan]

zip and/or tar support needs checking too.

[grynspan]

#define	RTLD_DEFAULT	((void *) -2)	/* Use default search algorithm. */

[grynspan]

Tracked internally as rdar://142344821.