pru: Fix memory corruption on syscall return

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>