Mac: Explicitly set architecture to selected tool chain.

The tool chain selection in build configurations had no effect on the
actual build, which is especially visible on Mac with universally built
Qt. The patch passes the tool chain's architecture (+bit width)
explicitly to Qmake. Users can override this behavior as usual by adding
or removing architecture flags from CONFIG in the additional arguments
field.

Task-number: QTCREATORBUG-6088
Change-Id: I92fdb7d5650db6e681945fa82e86422917f84035
Reviewed-by: Tobias Hunger <tobias.hunger@nokia.com>