This significantly simplifies the implementation, properly handles
C vs C++ compilers, and defaults to 64-bit architectures for iOS devices
and simulators.

LLVM-GCC support has been removed, as the last version to support it was
4.6.3, which is far below the version of Xcode we still intend to
support. The toolchains are also given more sensible names
("Apple Clang") instead of iphonesimulator-clang and iphoneos-clang, as
the there is now only one compiler for all Apple platform targets, and
there is one entry of the Apple Clang compiler per supported
architecture.

The different to the end user is minimal except that the compilers are
named better. The number of kits and their configurations remain the
same, but this patch paves the way for tvOS and watchOS support.

Change-Id: I6e2b17f8c17d9dd504f0ad540e08782f291b73c0
Reviewed-by: Vikas Pachdha <vikas.pachdha@qt.io>