libnetbsd: import __CTASSERT(..) macros

These compile-time assert macros are similar to Static_assert on FreeBSD.

These macros are in use in newer versions of contrib/netbsd-tests.

Obtained from: https://github.com/NetBSD/src (c26cc77b3a0b26b95a2)
MFC after: 1 week

(cherry picked from commit d1f1402dff63dd163d4ba4ac65f7f2ffd00c5e08)