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.

(Imported from upstream's 82e089308bd9a7794a45f0fa3973d7659420fbd8.)

Change-Id: Ie9708bb521c8d7d01afd2e064576f46be2a811a5
Reviewed-on: https://boringssl-review.googlesource.com/12821
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>
Reviewed-by: Steven Valdez <svaldez@google.com>
Reviewed-by: David Benjamin <davidben@google.com>
Commit-Queue: David Benjamin <davidben@google.com>

crypto/perlasm/x86_64-xlate.pl

@@ -257,11 +257,18 @@ my %globals;
 	$self->{base}  =~ s/^[er](.?[0-9xpi])[d]?$/r\1/;
 
 	# Solaris /usr/ccs/bin/as can't handle multiplications
-	# in $self->{label}, new gas requires sign extension...
+	# in $self->{label}...
 	use integer;
 	$self->{label} =~ s/(?<![\w\$\.])(0x?[0-9a-f]+)/oct($1)/egi;
 	$self->{label} =~ s/\b([0-9]+\s*[\*\/\%]\s*[0-9]+)\b/eval($1)/eg;
-	$self->{label} =~ s/\b([0-9]+)\b/$1<<32>>32/eg;
+
+	# Some assemblers insist on signed presentation of 32-bit
+	# offsets, but sign extension is a tricky business in perl...
+	if ((1<<31)<<1) {
+	    $self->{label} =~ s/\b([0-9]+)\b/$1<<32>>32/eg;
+	} else {
+	    $self->{label} =~ s/\b([0-9]+)\b/$1>>0/eg;
+	}
 
 	if (!$self->{label} && $self->{index} && $self->{scale}==1 &&
 	    $self->{base} =~ /(rbp|r13)/) {