Replace cc, c++ & cpp with symlinks

Replace cc, c++ & cpp with symlinks

In case of installation of both clang and GCC, we want to be able to
switch which one is the default by adapting the cc, c++ & cpp symlinks.

The default behaviour implemented here is to prefer clang over gcc if
they are both installed.

Change-Id: Ic14720cd876d2bf934d345a955cb5789378209e6