Added the first version of per-architecture binaries for composition

Pros:
+ we can have prebuild versions for all the architectures supported
  by Vc (Amd XMA4 and XOP are not supported by Vc yet)
+ the implementation is chosen dynamically on Krita start
+ the semi-general code for multi-arch builds now in
  KoVcMultiArchBuildSupport.h (might be ported upstream in the future)

Cons:
- it depends on Vc's 'staging' branch, so it can't be put in master
  right now
- the code became much less readable due to all that template magic
- I had to copy-paste Vc's 'vc_compile_for_all_implementations' cmake
  macro, because we do not need 'Scalar' implementation
- the size of the pigment library grew almost 1.5 times: 11->17 MiB
  (probably, we still need plugin system for this)

CCMAIL:kimageshop@kde.org
CCMAIL:kretz@kde.org