[Gc] iOS ARM64 (Aarch64) support

Hi,

We've made a first attempt [1] at porting the GC to iOS ARM64
(Aarch64) and so far it seems to be working fine. Wanted to check
whether someone else is working on this as well?

One initial problem we had was what we'd set STACKBOTTOM to. The value
for iOS 32-bit seemed totally arbitrary. But AFAICS, for Darwin, the
actual value for STACKBOTTOM is ignored and instead it's set to the
expression (ptr_t)pthread_get_stackaddr_np(pthread_self()) in
os_dep.c. Can someone please confirm that this is indeed what happens
for OS X/iOS?

Please let me know whether you'd be interested in merging this and
I'll send a PR.

[1] https://github.com/robovm/bdwgc/commit/a583a080fd9bd57927190dfa090ad281ddc931d5

Regards,
Niklas
RoboVM