Add macros for assuming and unreachable code

Add macros for assuming and unreachable code

Use these macros to tell the compiler about conditions that may
happen, so it will generate better code. But do not assume that they
will do anything special.

Change-Id: I89ec4f65f48a9340ccf5ffc4ae4b8c3d8897c8b1
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@nokia.com>
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>