nvi: Replace Clang-only __builtin_is_aligned with C code (#124)

We should use alignof in the future.

Obtained from: https://github.com/lichray/nvi2/commit/25c4d7db4ea638a31ac458b733a3b67b0a0ff634

(cherry picked from commit 06a98fefd3d3ff42b7e7832af6c0736b98f167ac)