x86_64-xlate.pl: import fix(?) from upstream.

This imports the changes to x86_64-xlate from upstream's
9c940446f6. It looks like it's a fix,
although it doesn't alter our generated asm at all. Either way, no point
in diverging from upstream on this point.

Change-Id: Iaedf2cdb9580cfccf6380dbc3df36b0e9c148d1c
Reviewed-on: https://boringssl-review.googlesource.com/13767
Commit-Queue: Adam Langley <agl@google.com>
Commit-Queue: David Benjamin <davidben@google.com>
Reviewed-by: David Benjamin <davidben@google.com>

crypto/perlasm/x86_64-xlate.pl

@@ -1047,7 +1047,7 @@ my $rdrand = sub {
       my @opcode=();
       my $dst=$1;
 	if ($dst !~ /[0-9]+/) { $dst = $regrm{"%e$dst"}; }
-	rex(\@opcode,0,$1,8);
+	rex(\@opcode,0,$dst,8);
 	push @opcode,0x0f,0xc7,0xf0|($dst&7);
 	@opcode;
     } else {
@@ -1060,7 +1060,7 @@ my $rdseed = sub {
       my @opcode=();
       my $dst=$1;
 	if ($dst !~ /[0-9]+/) { $dst = $regrm{"%e$dst"}; }
-	rex(\@opcode,0,$1,8);
+	rex(\@opcode,0,$dst,8);
 	push @opcode,0x0f,0xc7,0xf8|($dst&7);
 	@opcode;
     } else {