asm-poly1305-aarch64: fix building with clang

* cipher/asm-poly1305-aarch64.h (POLY1305_BLOCK_PART25): Use correct
instruction format for right-shifting.

GCC recognizes 32-bit ARM's 'move with barrel shift operand' on Aarch64,
but clang does not. Therefore switch to use shift instruction instead.

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