Capture a backtrace before calling wasm (#830)

* Capture a backtrace before calling wasm

This helps mitigate the issue, at least locally, described in #829 and
there's some more comments inline in the code as well.

* Run rustfmt

* Move around where the trace happens

crates/runtime/src/signalhandlers.rs

@@ -61,6 +61,17 @@ pub extern "C" fn wasmtime_init_eager() {
     state.tried = true;
     assert!(!state.success);
 
+    // This is a really weird and unfortunate function call. For all the gory
+    // details see #829, but the tl;dr; is that in a trap handler we have 2
+    // pages of stack space on Linux, and calling into libunwind which triggers
+    // the dynamic loader blows the stack.
+    //
+    // This is a dumb hack to work around this system-specific issue by
+    // capturing a backtrace once in the lifetime of a process to ensure that
+    // when we capture a backtrace in the trap handler all caches are primed,
+    // aka the dynamic loader has resolved all the relevant symbols.
+    drop(backtrace::Backtrace::new_unresolved());
+
     if unsafe { EnsureEagerSignalHandlers() == 0 } {
         return;
     }