Enable precompiled headers on iOS, tvOS, watchOS

The actual blocker for precompiled headers is not the iOS/tvOS/watchOS
platforms, but the way qmake handled multiple-architecture builds on
Apple platforms.

This patch allows multi-arch builds to be performed while using
precompiled headers.

Since df91ef3d6c55692a0236f67b6c6b134a3bf84098 (April 2009), Clang has
had support for PCH files in the driver, which allows to use the
-include flag to automatically translate to -include-pch. We can then
take advantage of the fact that the -include option is allowed to not
be separate from its argument, which lets us take advantage of -Xarch to
specify a per-architecture precompiled header file.

This is done through some magic in the qmake Makefile generator which
"multiplexes" the PCH creation rule across multiple architectures and
replaces a series of tokens with the proper precompiled header paths
and architecture flags at usage point.

Change-Id: I76c8dc9cda7e218869c2919f023d9b04f311c6fd
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@theqtcompany.com>

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