iOS arm64 support coming along nicely

[Dan Olson via digitalmars-d-ldc]

Having a free day on Monday and a recently acquired iPhone 6, I started
adding support for iOS arm64 to LDC.  So far going smoothly with
druntime compiling, Threads and Fiber switching working on the h/w.  TLS
functions with same approach as 32-bit ARM: a small modification to LLVM
that works with [1].

Next task is to fix up the C ABI as iOS calling convention [2] is a
variation on AAPCS64.  Started with LDC's gen/abi-aarch64.cpp, but a
couple files of phobos ICE in LLVM vararg code, which I expect is due to
iOS variation.

[1] https://github.com/smolt/iphoneos-apple-support 
[2] https://developer.apple.com/library/ios/documentation/Xcode/Conceptual/iPhoneOSABIReference/Articles/ARM64FunctionCallingConventions.html

-- 
Dan