[SIGNAL] Fixed my_signal not reporting sigaction failure for SIGKILL/SIGSTOP

[1362525207]

Problem: Test case failure for LTP signal01:
signal01.c:54: TFAIL: (long)signal(SIGKILL, tc->sighandler) succeeded

Reason: 'my_signal' ignored the return value of the host 'sigaction()' and always returned 'oldact.sa_handler'. For uncatchable signals (SIGKILL/SIGSTOP) the kernel rejects the handler with EINVAL, but the guest saw a non-'SIG_ERR' result and assumed success.

Now returns 'SIG_ERR' on failure. This matches the signal() contract and glibc's own implementation, which also returns SIG_ERR when sigaction fails:

  act.sa_flags = __sigismember (&_sigintr, sig) ? 0 : SA_RESTART;
  if (__sigaction (sig, &act, &oact) < 0)
    return SIG_ERR;

  return oact.sa_handler;

Validation:

• ctest: 34/34 passed on RV64 hardware.
• LTP signal01: 6/6 (was 5/6).