Skip stack overflow handler for panic=immediate-abort

std installs guard pages and a signal handler to ensure that stackoverflows 1) terminate abruptly and 2) print an nice message. Even for panic=immediate-abort, 1) is desirable, we don't want silent data corruption there. But 2) is completely unnecessary, as users deliberately *don't* want nice messages, they want minimum binary size.

Therefore, skip the entire guard signal handler setup, which saves a lot of bytes.

I tested this with a hello world binary using fat LTO, build-std, panic=immediate-abort, opt-level=s, strip=debuginfo.

`size` reports significant savings:

```
   text	   data	    bss	    dec	    hex	filename
  15252	   1032	    104	  16388	   4004	tiny-before
   6881	    964	     48	   7893	   1ed5	tiny-after2
```

`nm -U` goes from 71 to 56, getting rid of a bunch of stack overflow related symbols. The disk size goes from `31k` to `24k`.

The impact on the error message is minimal, as the message was already
missing.

before:
```
fish: Job 1, './tiny-so-before' terminated by signal SIGABRT (Abort)
```

after:
```
fish: Job 1, './tiny-so-after' terminated by signal SIGSEGV (Address boundary error)
```

2 files changed, 13 insertions, 2 deletions

diff --git a/library/std/src/sys/pal/unix/stack_overflow.rs b/library/std/src/sys/pal/unix/stack_overflow.rs
index 0d2100d66bc..08a3cee64ba 100644
--- a/library/std/src/sys/pal/unix/stack_overflow.rs
+++ b/library/std/src/sys/pal/unix/stack_overflow.rs
@@ -148,6 +148,13 @@ mod imp {
 
         let mut guard_page_range = unsafe { install_main_guard() };
 
+        // Even for panic=immediate-abort, installing the guard pages is important for soundness.
+        // That said, we do not care about giving nice stackoverflow messages via our custom
+        // signal handler, just exit early and let the user enjoy the segfault.
+        if cfg!(panic = "immediate-abort") {
+            return;
+        }
+
         // SAFETY: assuming all platforms define struct sigaction as "zero-initializable"
         let mut action: sigaction = unsafe { mem::zeroed() };
         for &signal in &[SIGSEGV, SIGBUS] {
@@ -179,6 +186,9 @@ mod imp {
     /// Must be called only once
     #[forbid(unsafe_op_in_unsafe_fn)]
     pub unsafe fn cleanup() {
+        if cfg!(panic = "immediate-abort") {
+            return;
+        }
         // FIXME: I probably cause more bugs than I'm worth!
         // see https://github.com/rust-lang/rust/issues/111272
         unsafe { drop_handler(MAIN_ALTSTACK.load(Ordering::Relaxed)) };
@@ -230,7 +240,7 @@ mod imp {
     /// Mutates the alternate signal stack
     #[forbid(unsafe_op_in_unsafe_fn)]
     pub unsafe fn make_handler(main_thread: bool, thread_name: Option<Box<str>>) -> Handler {
-        if !NEED_ALTSTACK.load(Ordering::Acquire) {
+        if cfg!(panic = "immediate-abort") || !NEED_ALTSTACK.load(Ordering::Acquire) {
             return Handler::null();
         }
 
diff --git a/library/std/src/sys/pal/windows/mod.rs b/library/std/src/sys/pal/windows/mod.rs
index 3357946b8f7..b7578b01584 100644
--- a/library/std/src/sys/pal/windows/mod.rs
+++ b/library/std/src/sys/pal/windows/mod.rs
@@ -22,7 +22,8 @@ pub mod os;
 pub mod pipe;
 pub mod time;
 cfg_select! {
-    not(target_vendor = "uwp") => {
+    // We don't care about printing nice error messages for panic=immediate-abort
+    all(not(target_vendor = "uwp"), not(panic = "immediate-abort")) => {
         pub mod stack_overflow;
     }
     _ => {