Mac does not allow clearing sigaltstack after abnormal signal exit

If we have a crash in a signal handler and we then exit DR we get this on Mac:

```
-22 for 0x4f069000 0xe000 0x4 cur=0x4f04eb80
```

The app crash is inside the handler: does the kernel really track whether
we called sigreturn? It looks like it does:

```
/extsw/mac/xnu-2422.1.72/bsd/kern/kern_sig.c
if (ss.ss_flags & SA_DISABLE) {
/* if we are here we are not in the signal handler ;so no need to check */
if (uth->uu_sigstk.ss_flags & SA_ONSTACK)
return (EINVAL);

/extsw/mac/xnu-2422.1.72/bsd/dev/i386/unix_signal.c
sendsig()
ut->uu_sigstk.ss_flags |= SA_ONSTACK;
sigreturn()
ut->uu_sigstk.ss_flags &= ~SA_ONSTACK;
```

This doesn't seem like the right thing for the kernel to do, but we'll have
to work around it by allowing the syscall to fail.