x86_64 assembly pack: tune clang version detection.

RT#4142

Reviewed-by: Richard Levitte <levitte@openssl.org>
(cherry picked from commit 76eba0d9

)

Resolved conflicts:
	crypto/bn/asm/x86_64-mont.pl
	crypto/bn/asm/x86_64-mont5.pl

Reviewed-by: Kurt Roeckx <kurt@openssl.org>