Add __unused and __used macros that expand to appropriate __attribute__ expressions depending on the GCC version in use.  Use __unused to supress warnings from GCC's -Wunused options, and use __used to prevent code from being optimized away.  From NetBSD.
Add __unused and __used macros that expand to appropriate __attribute__
expressions depending on the GCC version in use.  Use __unused to supress
warnings from GCC's -Wunused options, and use __used to prevent code from
being optimized away.  From NetBSD.

ok tedu@, marco@

orumin/openbsd-efivars,orumin/openbsd-efivars,orumin/openbsd-efivars,orumin/openbsd-efivars