Proposal: packed TLS for ELF

Never heard back from Walter about the dmd patch I used for Android/x86 and I know he's very careful about copyright, so I'm submitting it as a pull request.  Right now, I'm just looking to find out if some sort of packed TLS for ELF patch like this would be considered for merging.  If so, I'll clean this patch up and figure out a way to work it into the existing build, but I don't want to do that cleanup if the basic concept is not deemed worthwhile for dmd.  I used this patch to compile druntime and phobos for Android/x86, with almost all modules' unit tests passing.

Let me know if this sort of packed TLS for ELF approach would be worth merging and I'll clean up this pull.