Log Message:
libnbcompat: Update to 20230904.

Pull in changes from revision 1.13 of NetBSD sha2.c from 14 years ago to
fix type punning issues seen with newer GCCs.

Fixes "pkg_admin digest" on SmartOS with GCC 12, where the output was
completely wrong, causing bulk builds to rebuild every package every time
now that USE_PKG_ADMIN_DIGEST=yes is the default.