bitbake.conf: Start to separate out gcc related variable definitions

bitbake.conf: Start to separate out gcc related variable definitions

To be able to switch toolchains, we need to separate out the gcc definitions
into seperate include files. This patch starts that process. Whilst the
include is still hardcoded for now, it allows developers to start experimenting
with this locally more easily and stops people reinventing this patch. A
sample clang configuruation is also included which I was using for experimentation.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>