Auto merge of #11530 - jdm:sigsegv, r=metajack

Obtain backtraces automatically from segfaults

<!-- Please describe your changes on the following line: -->
This enables more meaningful output from observing hard crashes outside of GDB through the judicious use of a SIGSEGV signal handler.

---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `__` with appropriate data: -->
- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix #9371 (github issue number if applicable).
- [X] There are tests for these changes OR

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components/script/dom/testbinding.rs

@@ -571,6 +571,15 @@ impl TestBindingMethods for TestBinding {
     fn FuncControlledAttributeEnabled(&self) -> bool { false }
     fn FuncControlledMethodDisabled(&self) {}
     fn FuncControlledMethodEnabled(&self) {}
+
+    #[allow(unsafe_code)]
+    fn CrashHard(&self) {
+        static READ_ONLY_VALUE: i32 = 0;
+        unsafe {
+            let p: *mut u32 = &READ_ONLY_VALUE as *const _ as *mut _;
+            ptr::write_volatile(p, 0xbaadc0de);
+        }
+    }
 }
 
 impl TestBinding {

components/script/dom/webidls/TestBinding.webidl

@@ -456,3 +456,8 @@ interface TestBinding {
   [Func="TestBinding::condition_satisfied"]
   const unsigned short funcControlledConstEnabled = 0;
 };
+
+partial interface TestBinding {
+  [Pref="dom.testable_crash.enabled"]
+  void crashHard();
+};