Add arm64 arch to the iOS boost static libraries

These are compiled according to the [instructions](http://libcinder.org/docs/v0.8.5/_cinder_boost.html) at libcinder.org, with the addition of `-arch arm64`.

With this `user-config.jam`:
```
using clang : ios
: xcrun clang -arch armv7 -arch armv7s -arch arm64 -stdlib=libc++ -std=c++11 -miphoneos-version-min=5.0 -fvisibility-inlines-hidden -isysroot /Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS7.1.sdk/
;
```
```bash
./bootstrap.sh --with-libraries=filesystem,system,date_time
rm -rf stage && ./b2 -a -j9 toolset=clang-ios link=static stage
```
I've tested all the iOS samples. The ones that worked with the previous libs work with these.