poly1305: silence compiler warning on clang/aarch64

* cipher/poly1305.c (MUL_MOD_1305_64): cast zero constant to 64-bits.

This patch fixes "value size does not match register size specified
by the constraint and modifier [-Wasm-operand-widths]" warnings when
building with clang/aarch64.

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