build: Add generic compiler builtins support

Wrap compiler builtins into shared functions with proper generic
implementations. __builtin_clz() isn't wrapped for now because its use
by screenshooter is pretty specific. It will be properly wrapped in
the next commit which needs a round up to the next power of 2
function.

Signed-off-by: Loïc Molinari <loic.molinari@collabora.com>