Support cross-compiling for iOS (armv7)

Apple rejected this patch because it's Apple. They provide 2 different functions  pthread_getspecific (public one) and pthread_getspecific_direct (this one is private and inlined) and the actual their fear is that if in any moment in future they'll need to change the method - the code using it directly will be updated and rebuilt. 

In case of inlined function it is not a problem as it is marked as private and considering this it is expected to work only in Apple products so they will be 100% recompiled

In case of public function it's obvious that if they provide a new library - all client code continues to work without problem.

Let's estimate the probability of changing method for setting thread-specific values in armv7...
I'd say that it is quite low (almost zero) and the main reason is that armv7 is dead end for Apple devices, they do switch to Arm64 quite extensively and I'd say that armv7 devices will live max 2 years more. It is a bright side for developer of how fast Apple eliminates "old" hardware. Considering this I can't imagine what reason can force them to change method as it was used since 2009 (may be even earlier, but I've stopped tracing there). 

I'll try to find someone with installed iOS8 beta to check that they haven't changed method there though and if not - I'd say it's quite safe and that there should be an opportunity to turn them on for safety.