perlasm/x86_64-xlate.pl: refine sign extension in ea package.

$1<<32>>32 worked fine with either 32- or 64-bit perl for a good while,
relying on quirk that [pure] 32-bit perl performed it as $1<<0>>0. But
this apparently changed in some version past minimally required 5.10,
and operation result became 0. Yet, it went unnoticed for another while,
because most perl package providers configure their packages with
-Duse64bitint option.

Reviewed-by: Rich Salz <rsalz@openssl.org>