rijndael-vaes-avx512: add PIC @PLT for AVX2 function jumps

* cipher/asm-common-amd64.h (AT_PLT): New.
* cipher/rijndael-vaes-avx512-amd64.S: Use AT_PLT for jumps to
AVX2/VAES functions.

• GnuPG-bug-id: T8071
• Signed-off-by: Jussi Kivilinna <jussi.kivilinna@iki.fi>