[illumos] enable SIGSEGV handler to detect stack overflows

Use the same code as Solaris. I couldn't find any tests regarding this, but I
did test a stage0 build against my stack-exhaust-test binary [1]. Before:

```
running with use_stacker = No, new_thread = false, make_large_local = false
zsh: segmentation fault (core dumped)  cargo run
```

After:

```
running with use_stacker = No, new_thread = false, make_large_local = false

thread 'main' has overflowed its stack
fatal runtime error: stack overflow
zsh: IOT instruction (core dumped)  cargo +stage0 run
```

Fixes rust-lang#128568.

[1] https://github.com/sunshowers/stack-exhaust-test/

std/src/sys/pal/unix/stack_overflow.rs

@@ -32,7 +32,8 @@ impl Drop for Handler {
     target_os = "macos",
     target_os = "netbsd",
     target_os = "openbsd",
-    target_os = "solaris"
+    target_os = "solaris",
+    target_os = "illumos",
 ))]
 mod imp {
     #[cfg(not(all(target_os = "linux", target_env = "gnu")))]
@@ -280,7 +281,7 @@ mod imp {
         libc::SIGSTKSZ
     }
 
-    #[cfg(target_os = "solaris")]
+    #[cfg(any(target_os = "solaris", target_os = "illumos"))]
     unsafe fn get_stack_start() -> Option<*mut libc::c_void> {
         let mut current_stack: libc::stack_t = crate::mem::zeroed();
         assert_eq!(libc::stack_getbounds(&mut current_stack), 0);
@@ -486,7 +487,12 @@ mod imp {
         Some(guardaddr..guardaddr + page_size)
     }
 
-    #[cfg(any(target_os = "macos", target_os = "openbsd", target_os = "solaris"))]
+    #[cfg(any(
+        target_os = "macos",
+        target_os = "openbsd",
+        target_os = "solaris",
+        target_os = "illumos",
+    ))]
     // FIXME: I am probably not unsafe.
     unsafe fn current_guard() -> Option<Range<usize>> {
         let stackptr = get_stack_start()?;
@@ -569,7 +575,8 @@ mod imp {
     target_os = "macos",
     target_os = "netbsd",
     target_os = "openbsd",
-    target_os = "solaris"
+    target_os = "solaris",
+    target_os = "illumos",
 )))]
 mod imp {
     pub unsafe fn init() {}