swift234

Linux install tarbal mechanics

A couple of small changes to make linux install/archiving work.

- Just use `tar`. There is no `gnutar` on Linux and `tar` on OS X
  works the same with the options we use.
- Allow cmark to be optionally installed (probably not by default)
- Set the executable format to "ELF" in the CMake configs so the
  ELF utilities built into CMake will be invoked (this isn't
  automatically done for Swift because CMake doesn't know what
  Swift is yet).

rdar://problem/22429445

Swift SVN r31618