MINOR: compiler: implement an ONLY_ONCE() macro

MINOR: compiler: implement an ONLY_ONCE() macro

There are regularly places, especially in config analysis, where we
need to report certain things (warnings or errors) only once, but
where implementing a counter is sufficiently deterrent so that it's
not done.

Let's add a simple ONLY_ONCE() macro that implements a static variable
(char) which is atomically turned on, and returns true if it's set for
the first time. This uses fairly compact code, a single byte of BSS
and is thread-safe. There are probably a number of places in the config
parser where this could be used. It may also be used to implement a
WARN_ON() similar to BUG_ON() but which would only warn once.