add warning control macro set

add warning control macro set

Add a way to toggle on and off a warning for a specific section of code.
This supports clang and gcc, and has no effect for any other compilers.

This follows commit bfc29ba442dbb65599f29fe5aa44c6219ed0d3a8 and
provides a generic way to handle warnings in such corner cases.

To disable a warning enabled by "-Wsome-warning" for a specific code:
  WARN_OFF(some-warning)  // disable compiler warning
  ; /* some code */
  WARN_ON(some-warning)   // enable the warning again

Change-Id: I0101caa0aa775e2b905c7b3b5fef3bbdce281673
Signed-off-by: Gabriel Ganne <gabriel.ganne@enea.com>