camellia-aarch64: do not store/load link register to/from stack

* cipher/camellia-aarch64.S (_gcry_camellia_arm_encrypt_block)
(_gcry_camellia_arm_decrypt_block): Store/load dummy x20 instead
of x30.

• GnuPG-bug-id: T7220
• Signed-off-by: Jussi Kivilinna <jussi.kivilinna@iki.fi>