aes-gcm-avx512.pl: fix non-reproducibility issue

Replace the random suffix with a counter, to make the
build reproducible.

Fixes #20954
Reviewed-by: NRichard Levitte <levitte@openssl.org>
Reviewed-by: NMatthias St. Pierre <Matthias.St.Pierre@ncp-e.com>
Reviewed-by: NTom Cosgrove <tom.cosgrove@arm.com>
Reviewed-by: NHugo Landau <hlandau@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/22415)