CentOS 6 distro detection

Hi,

Currently CentOS 6 isn't detected as RHEL6 linux distro.

Fix:

Index: lib/Driver/ToolChains.cpp

This should be refactored so that it looks like

if (RHEL OR CentOS) {
  figure out the version.
}

instead of repeating the RHEL or CentOS check for each version.