PATCH: Add objc_thread_add and objc_thread_remove to libobjc

[Stan Shebs]

Nicola Pero wrote:
> 
> here are two functions which can be used to have Objective-C threads
> cooperate with threads started from alien environments (eg, to allow
> Objective-C calls being made from arbitrary Java threads without causing
> thread problems to the objc runtime). [...]

I've committed these to libobjc, both to mainline and 3.0 branch,
pretty much verbatim, except that I tweaked the ChangeLog entry to
be a little more standard in form.  Thanks for the contribution!

(JIGS looks pretty cool, BTW)

Stan