Mono's GC fails on sigaltstack w/ SS_DISABLE
Repro steps

```
Bin/ELFLoader -s -c irjit -n 500 -- /usr/bin/mono /usr/lib/mono/4.5/mcs.exe
error CS2008: No files to compile were specified
Compilation failed: 1 error(s), 0 warnings
Mono: GC_MAJOR: (user request) time 50.11ms, stw 77.87ms los size: 0K in use: 0K
Mono: GC_MAJOR_SWEEP: major size: 768K in use: 81K
* Assertion at mini-exceptions.c:3191, condition `err == 0' not met
```

Mono Source: https://github.com/mono/mono/blob/master/mono/mini/mini-exceptions.c#L3196
```
mono_free_altstack (MonoJitTlsData *tls)
{
    stack_t sa;
    int err;

    sa.ss_sp = tls->signal_stack;
    sa.ss_size = MONO_ARCH_SIGNAL_STACK_SIZE;
    sa.ss_flags = SS_DISABLE;
    err = sigaltstack  (&sa, NULL);
    g_assert (err == 0);
```