ABI: Improve tool chain selection for Qt projects

* Set a null-ABI for toolchains where detection of ABI failed: This
  allows the user to select any tool chain.

* Warn if ABI detection fails by showing a message in the Qt version
  setup dialog

* Do not override the mkspec if the Qt version has a null ABI only:
  We (have to) trust the user to know what he is doing in that case.

* GCC: Add version information to GCC tool chain and use it to select
  the mkspec on Mac.

* GCC: Do not override mkspec if we do a cross-compile
  (and actually detect that).

* GCC: Leave out -32 or -64 from mkspec if the compiler binaries ABI
  matches the target ABI.

* Linux-generic is compatible with all other linuxes (both ways)
  The targets themselves do additional filtering and will remove
  anything that does not work for them anyway.

Task-number: QTCREATORBUG-5756
Task-number: QTCREATORBUG-5960
Change-Id: Ib3c0ac91f3aa8e2a823e3e9891a02bdccb97af14
Reviewed-on: http://codereview.qt-project.org/4281


Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Daniel Teske <daniel.teske@nokia.com>