RISC-V: Update shift-shift-5.c testcase for big endian

gcc/testsuite/

	* gcc.target/riscv/shift-shift-5.c (sub): Change
	order of struct fields depending on byteorder.

gcc/testsuite/gcc.target/riscv/shift-shift-5.c

@@ -7,7 +7,11 @@ unsigned long
 sub (long l)
 {
   union u {
+#if __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__
     struct s { int a : 19; unsigned int b : 13; int x; } s;
+#else
+    struct s { int x; unsigned int b : 13; int a : 19; } s;
+#endif
     long l;
   } u;
   u.l = l;