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

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

Fixes #20954

Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Matthias St. Pierre <Matthias.St.Pierre@ncp-e.com>
Reviewed-by: Tom Cosgrove <tom.cosgrove@arm.com>
Reviewed-by: Hugo Landau <hlandau@openssl.org>
(Merged from #22415)

(cherry picked from commit 0fbc50e)