Add a signal handler for macos and linux.

Implementation Notes:
- To avoid setjmp, longjmp, and the mess that those create, we instead set the interrupting
    context of the signal handler to return into the `throw_trap` routine. To my surprise,
    this actually works. The stack ends up getting unwound normally and the memory-oob error
    is caught by the trampoline.

lib/llvm-backend/src/backend.rs

@@ -243,6 +243,10 @@ impl LLVMBackend {
             )
         };
 
+        unsafe {
+            crate::platform::install_signal_handler();
+        }
+
         if res != LLVMResult::OK {
             panic!("failed to load object")
         }