Add support for a custom built dynamic loader

- Add _PatchELF_ utility for patching ELF binaries in post-install
  step.
- Do ELF relocation for `bin/$SCRAM_ARCH` and `test/$SCRAM_ARCH` using
  `CMS_INSTALL_PREFIX`.
- Relocation is only done for slc6_amd64_* (current production
  architecture for CMSSW_7_2_X).

This makes `IB/CMSSW_7_2_X/stable` and `IB/CMSSW_7_2_X/next` identical. 

Signed-off-by: David Abdurachmanov <David.Abdurachmanov@cern.ch>