compiler.h: Explain how __is_constexpr() works

The __is_constexpr() macro is dark magic. Shed some light on it with
a comment to explain how and why it works.

Acked-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Link: https://lore.kernel.org/r/20240301044428.work.411-kees@kernel.org


Signed-off-by: Kees Cook <keescook@chromium.org>