atomic.h: Work around gcc spurious "unused value" warning

atomic.h: Work around gcc spurious "unused value" warning

Some versions of gcc produce a spurious warning if the result of
__atomic_compare_echange_n() is not used and the type involved
is a signed 8 bit value:
  error: value computed is not used [-Werror=unused-value]
This has been seen on at least
 gcc (Ubuntu 5.4.0-6ubuntu1~16.04.9) 5.4.0 20160609

Work around this by using an explicit cast to void to indicate
that we don't care about the return value.

We don't currently use our atomic_cmpxchg() macro on any signed
8 bit types, but the upcoming support for the Arm v8.1-Atomics
will require it.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>