Upstream TLS support

[jacob-hughes]

I noticed that you were interested in potentially upstreaming some changes I made in my own fork of BDWGC which enables compiler generated (i.e. non-POSIX) TLS support. This stuff is obviously highly non-portable and I doubt that it would work on anything other than x86_64 Linux. If that's ok with you, I'd be happy to tidy it up, ifdef it behind some build flags, and submit a PR?

Thanks!

[ivmai]

I think it is OK to add support just for linux x86_64, anyway there are many tls variants supported in bdwgc. Somewhere in bdwgc readme it is written that scanning pointers in tls is in todo list...

In low priority. Guarded with ifdef. No need to modify build scripts, just document the macro in README.macros.

[ivmai]

Probably Rust users might need it