Use _Static_assert() if available

Use _Static_assert() if available

The current compiletime_assert() doesn't work properly if code
optimization is not enabled, causing all compile time asserts to fail
regardless of the condition. This is because the compiler doesn't
realize that the external function will never get called for the
false case.

Use _Static_assert() if available for compiletime_assert(), and
fallback to the old method if we don't have it.

Signed-off-by: Jens Axboe <axboe@fb.com>