Add patched backtrace crate

- Get rid of RtlCaptureContext import, manually implement it
- Add support for older dbghelp versions. With this, backtraces on XP+
  are supported out of the box. It seems that the "best" version for old
  systems is the one from VS2005, found in
  "Microsoft Visual Studio 8\Common7\IDE\dbghelp.dll".
  This one is compiled with 9x support (unlike the VS2003 one!).

Author: seritools

.gitmodules

@@ -33,7 +33,7 @@
 	shallow = true
 [submodule "library/backtrace"]
 	path = library/backtrace
-	url = https://github.com/rust-lang/backtrace-rs.git
+	url = https://github.com/rust9x/backtrace-rs.git
 	shallow = true
 [submodule "src/tools/rustc-perf"]
 	path = src/tools/rustc-perf