Fix invalid link to C++ Exception Handling ABI documentation

Author: tjhu

src/libpanic_unwind/gcc.rs

@@ -4,7 +4,7 @@
 //! "Exception Handling in LLVM" (llvm.org/docs/ExceptionHandling.html) and
 //! documents linked from it.
 //! These are also good reads:
-//!     http://mentorembedded.github.io/cxx-abi/abi-eh.html
+//!     https://itanium-cxx-abi.github.io/cxx-abi/abi-eh.html
 //!     http://monoinfinito.wordpress.com/series/exception-handling-in-c/
 //!     http://www.airs.com/blog/index.php?s=exception+frames
 //!