Use _Static_assert on all platforms

This macro is part of the C11 standard.  Remove the use of a specialized macro
just for Zephyr, and use the standard macro for all platforms.  This reduces
dependencies on specific versions of Zephyr.

In addition.  The Zephyr-specific code was using toolchain/gcc.h which
would only work with gcc.h, so doesn't actually address any of the
objections.  If this ever becomes a problem with Zephyr, we can figure
out a proper way to fix this.

Signed-off-by: David Brown <david.brown@linaro.org>