[ios] - support Xcode 6.x and iOS 8 SDK

- use clang as default compiler for newer SDKs
- fakeroot: ignore interposing openat()
[ios] - fix libssh build with Xcode 6
[ios] - fix pil build with Xcode 6
[ios] - libRTV fix build for Xcode 6
[ios] - fix libmpeg2 build error

fallback asm code to C code
[ios/atv2] - adapt readme - we support Xcode 6.0.1 against iOS 8 SDK
[ios] - fix pvr addons build with Xcode 6