s390x assembly pack: fix bn_mul_comba4

Signed-off-by: NPatrick Steuer <patrick.steuer@de.ibm.com>
Reviewed-by: NPaul Dale <paul.dale@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/10454)

(cherry picked from commit 97a986f78289fef71bf8778dc4763458e983750c)

crypto/bn/asm/s390x.S

@@ -511,7 +511,7 @@ bn_mul_comba4:
 	lghi	zero,0
 
 	mul_add_c(0,0,c1,c2,c3);
-	stg	c1,0*8(%r3)
+	stg	c1,0*8(%r2)
 	lghi	c1,0
 
 	mul_add_c(0,1,c2,c3,c1);