iOS arm64 support coming along nicely

[Dan Olson via digitalmars-d-ldc]

arm64 iOS and iphone sim almost ready!  A few minor changes where needed
but now druntime/phobos unittests are looking good.  It is currently
based on 0.15.2 DMD frontend 2.066 [1].

Unfortunately I got a regression (unaligned load) with previously
working 32-bit armv7 after the varags change in 0.15.2.  I need to sort
that out now.

I haven't pushed necessary LLVM 3.6 arm64 changes yet to github.  Trying
to build ios-merge-release-0.15.2 without the updated LLVM will fail.

[1] https://github.com/smolt/ldc/tree/ios-merge-release-0.15.2

--
Dan