i#1569 AArch64: Fix drreg-test with double move to XZR.

The code in drreg-test.dll.c expects the MSR instruction to be
immediately preceded by the two immediate moves of the subtest number.
We can easily arrange that by moving to the zero register, XZR.

Review-URL: https://codereview.appspot.com/306470043

suite/tests/client-interface/drreg-test.c

@@ -325,9 +325,9 @@ GLOBAL_LABEL(FUNCNAME:)
         b        test5
         /* Test 5: fault aflags restore */
      test5:
-        movz     TEST_REG_ASM, DRREG_TEST_5_ASM
-        movz     TEST_REG_ASM, DRREG_TEST_5_ASM
         movz     TEST_REG_ASM, DRREG_TEST_AFLAGS_H_ASM, LSL 16
+        movz     xzr, DRREG_TEST_5_ASM
+        movz     xzr, DRREG_TEST_5_ASM
         msr      nzcv, TEST_REG_ASM
         nop
         mov      x0, HEX(0)