iOS: Enable fat builds containing both armv7 and arm64 slices

Apple will from February 1, 2015, require all applications uploaded to
the App Store to be built for both 32-bit (armv7/s) and 64-bit (arm64).

https://developer.apple.com/news/?id=10202014a

We enable fat Qt binaries by passing both -arch armv7 and -arch arm64
to clang, which takes care of lipoing together the two slices for each
object file. This unfortunately means twice the build time and twice
the binary size for our libraries.

Since precompiled headers are architecture specific, and the -Xarch
option can't be used with -include-pch, we need to disable precompiled
headers globally. This can be improved in the future by switching to
pretokenized headers (http://clang.llvm.org/docs/PTHInternals.html

).

Since we're enabling 64-bit ARM builds, we're also switching the
simulator builds from i386 to fat i386 and x86_64 builds, so that
we are able to test 64-bit builds using the simulator, but we're
keeping i386 as the architecture Qt is aware of when it's building
for simulator, as we need the CPU features to match the lowest
common denominator.

Change-Id: I277e60bddae549d24ca3c6301d842405180aded6
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>