Fix Darwin host identification

[Karlson2k]

This PR should replace hardcoded kernel architecture for Darwin iOS with real architecture detection.
Also kernel bitness should be fixed for Darwin iOS.
Additionally SysInfo::GetKernelBitness and SysInfo::GetKernelCpuFamily now cache values instead of detection each time.
OS X detection was checked locally, iOS detection should work according to http://blog.timac.org/?p=907 and source code of NXGetLocalArchInfo().
It wold be great if some darwin dev will be able to check it on real Arm64 device.

@Memphiz @jmarshallnz ?

[Memphiz]

Can check ios 32bit on monday earliest - i am back at home for ios 64Bit and atv2 on wednesday or if i miss my short time slot - the monday after...

[Karlson2k]

@Memphiz Don't waste your time on iOS 32bit - this will work for sure.
Check only 64bit OS detection for 32bit app.

[Memphiz]

Based on this - would you be able to fix https://github.com/xbmc/xbmc/blob/master/xbmc/utils/CPUInfo.cpp#L132 for ios aswell? atm the CPU model and vendor is returned as empty (as "machdep.cpu.brand_string" and "machdep.cpu.vendor" are not implemented on ios).

[Karlson2k]

Rebased
@Memphiz I can implement only basic CPU detection. iOS don't give you any detailed CPU information.

[Memphiz]

@Karlson2k this is the log from a 64bit ios device - is this ok?

http://pastebin.com/yHBg9ums

[Karlson2k]

@Memphiz Thanks a lot! Yes, it works!

[Karlson2k]

Everything works, let's check by jenkins

jenkins build this please

[Karlson2k]

jenkins is fine, android fail is unrelated.

[MartijnKaijser]

jenkins build this please