[ios] - add support for compiling with Xcode 6 and iOS 8 SDK

This fixes compilation for ios/atv2 with latest Xcode 6.0.1 and iOS 8 SDK. Thx @linusyang who did the major work on this in his arm64 branch.

The last commit here (fixing pvr addons) will be dropped whenever the fix was merged in xbmc-pvr-addons repo (PR sent).

@davilla any idea how to properly fix the asm of libmpeg2 instead of disabling?
This line is a dirty hack to override `platform_cflags` for `arm64` to remove some *unsed cflags* warnings. It is not required for `armv7`.