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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ports/cef/Cargo.lock

@@ -1909,6 +1909,7 @@ name = "servo"
 version = "0.0.1"
 dependencies = [
  "android_glue 0.1.3 (registry+https://github.com/rust-lang/crates.io-index)",
+ "backtrace 0.2.2 (registry+https://github.com/rust-lang/crates.io-index)",
  "browserhtml 0.1.7 (git+https://github.com/browserhtml/browserhtml?branch=gh-pages)",
  "canvas 0.0.1",
  "canvas_traits 0.0.1",
@@ -1935,6 +1936,7 @@ dependencies = [
  "script 0.0.1",
  "script_layout_interface 0.0.1",
  "script_traits 0.0.1",
+ "sig 0.1.1 (registry+https://github.com/rust-lang/crates.io-index)",
  "style 0.0.1",
  "url 1.1.1 (registry+https://github.com/rust-lang/crates.io-index)",
  "util 0.0.1",
@@ -2044,6 +2046,11 @@ dependencies = [
  "winapi-build 0.1.1 (registry+https://github.com/rust-lang/crates.io-index)",
 ]
 
+[[package]]
+name = "sig"
+version = "0.1.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+
 [[package]]
 name = "simd"
 version = "0.1.0"