Update the Kiwi-iOS target to build for x86_64 and arm64

`make install` will now build a fat library with all 5 iOS architectures, `i386`, `x86_64`, `armv7`, `armv7s` and `arm64`.

The Kiwi-iOS composite target needed to be updated to get the x86_64 target library build along side the others.

Building explicitly for the x86_64 simulator required a bit of trial and error, xcodebuild seems unable to build i386 and x86_64 at the same time, so I added step *specifically* for x86_64.

Restricting the VALID_ARCHS, ARCHS and IPHONEOS_DEPLOYMENT_TARGET for the 64bit simulator build seemed to do the trick.

`ARCHS='x86_64' VALID_ARCHS='x86_64' IPHONEOS_DEPLOYMENT_TARGET='7.0'`

The resulting library shows support for all 5 architectures:

```
xcrun -sdk iphoneos lipo -info libKiwi.a 
Architectures in the fat file: libKiwi.a are: i386 x86_64 armv7 armv7s arm64
```