Fix duplicate mulh test case in rv32m tests (#630)

Author: 404allen404

isa/rv32um/mulh.S

@@ -23,7 +23,7 @@ RVTEST_CODE_BEGIN
 
   TEST_RR_OP( 5,  mulh, 0x00000000, 0x00000000, 0xffff8000 );
   TEST_RR_OP( 6,  mulh, 0x00000000, 0x80000000, 0x00000000 );
-  TEST_RR_OP( 7,  mulh, 0x00000000, 0x80000000, 0x00000000 );
+  TEST_RR_OP( 7,  mulh, 0x00004000, 0x80000000, 0xffff8000 );
 
   TEST_RR_OP(30,  mulh, 0xffff0081, 0xaaaaaaab, 0x0002fe7d );
   TEST_RR_OP(31,  mulh, 0xffff0081, 0x0002fe7d, 0xaaaaaaab );