rtld: silence clang's unterminated-string-initialization warning

Since the initializer is used in other places where we can't just
replace it with a char-by-char initializer, this adds a macro for the
nonstring attribute (match the linuxkpi definition).

Reviewed by: emaste, jhb
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D52535

(cherry picked from commit 86f325711c07b723b85c031d581d0a5c1a0964f3)