pru: Fix memory corruption on syscall return

In the initial code I missed one level of pointer indirection. Instead
of storing errno in impure_data, _impure_ptr was corrupted.

Only simulator is impacted. Real targets have no OS and no syscalls.

This resolves a bunch of stdio cases from the GCC testsuite:
  FAIL->PASS: gcc.c-torture/execute/printf-2.c   -O0  execution test

Signed-off-by: Dimitar Dimitrov <dimitar@dinux.eu>

libgloss/pru/syscalls.S

@@ -42,6 +42,7 @@ __SC_ret:
 	/* Invert return code and store to errno (first int in _impure_ptr).  */
 	rsb	r14, r14, 0
 	ldi32	r1, _impure_ptr
+	lbbo	r1, r1, 0, 4
 	sbbo	r14, r1, 0, 4
 	/* Return -1 (for both int32_t or int64_t).  */
 	fill	r14, 8