Add support for Apple iOS and improve detection of arm architecture

I've added a new predef ```BOOST_OS_IOS``` for Apple iOS based on detection of the preprocessor macro ```__ENVIRONMENT_IPHONE_OS_VERSION_MIN_REQUIRED__```.

Additionally, I've extended the detection for ```BOOST_ARCH_ARM``` with a check for ```__arm64```, a preprocessor macro which is set by the Apple clang compiler for the new ARMv8 64bit architecture.

Both changes enable the use of ```boost::predef``` with projects targeting Apple iOS for 32 and 64bit arm architectures.

It would be great if this patch can make it into the next boost release!