Do not build 32-bit ARM assembly on Aarch64
* configure.ac: Build "serpent-armv7-neon", "salsa20-armv7-neon", "chacha20-armv7-neon", "sha512-armv7-neon" and "keccak-armv7-neon" only on 32-bit ARM.
- GnuPG-bug-id: T7220
- Signed-off-by: Jussi Kivilinna <jussi.kivilinna@iki.fi>