Add profiling support for clang

To be able to use it, you need to make sure that tool llvm-profdata is available in your system.

In Debian/Ubuntu based system this can achieved by executing the following:
sudo update-alternatives --install /usr/bin/clang clang /usr/bin/clang-3.5 60 --slave /usr/bin/clang++ clang++ /usr/bin/clang++-3.5 --slave /usr/bin/llvm-profdata llvm-profdata /usr/bin/llvm-profdata-3.5

No functional change
My makefile-fu is not great, but could we do something like:

profile_prepare = $(comp)-profile-prepare