blake2: fix RIP register access for AVX/AVX2 implementations

* cipher/blake2b-amd64-avx2.S: Use rRIP instead of (RIP).
* cipher/blake2s-amd64-avx.S: Use rRIP instead of (RIP).

• Reported-by: Konstantin Ryabitsev <konstantin@linuxfoundation.org>
• Signed-off-by: Jussi Kivilinna <jussi.kivilinna@iki.fi>