Post-install step for ELF patching on SCRAM projects

The following adds `patchelf` new external package, modifies `glibc` to match SLC6 in terms on source code and compilation flags, modifies LDFLAGS for C/C++ to support custom dynamic loader/interpreter.

Enables modifying dynamic loader on CMSSW. ELF objects are modified in `bin/$SCRAM_ARCH` and `test/$SCRAM_ARCH`, though we don't ship tests with CMSSW by default.

We might also want to remove `glibc-toolfile.spec` it was mainly needed for bash wrappers.