add basic coredump generation (#5868)

This change adds a basic coredump generation after a WebAssembly trap
was entered. The coredump includes rudimentary stack / process debugging
information.

A new CLI argument is added to enable coredump generation:
```
wasmtime --coredump-on-trap=/path/to/coredump/file module.wasm
```

See ./docs/examples-coredump.md for a working example.

Refs https://github.com/bytecodealliance/wasmtime/issues/5732

docs/examples-coredump.md

@@ -0,0 +1,62 @@
+# Using Wasm coredump
+
+The following steps describe how to debug using Wasm coredump in Wasmtime:
+
+1. Compile your WebAssembly with debug info enabled; for example: 
+
+    ```sh
+    $ rustc foo.rs --target=wasm32-wasi -C debuginfo=2
+    ```
+
+<details>
+    <summary>foo.rs</summary>
+
+    fn c(v: usize) {
+        a(v - 3);
+    }
+
+    fn b(v: usize) {
+        c(v - 3);
+    }
+
+    fn a(v: usize) {
+        b(v - 3);
+    }
+
+    pub fn main() {
+        a(10);
+    }
+</details>
+
+2. Run with Wasmtime and Wasm coredump enabled:
+
+    ```sh
+    $ wasmtime --coredump-on-trap=/tmp/coredump foo.wasm
+
+    thread 'main' panicked at 'attempt to subtract with overflow', foo.rs:10:7
+    note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
+    Error: failed to run main module `foo.wasm`
+
+    Caused by:
+        0: Core dumped at /tmp/coredump
+        1: failed to invoke command default
+        2: error while executing at wasm backtrace:
+                    ...
+    ```
+
+3. Use [wasmgdb] to debug:
+    ```sh
+    $ wasmgdb foo.wasm /tmp/coredump
+
+    wasmgdb> bt
+    ...
+    #13     000175 as panic () at library/core/src/panicking.rs
+    #12     000010 as a (v=???) at /path/to/foo.rs
+    #11     000009 as c (v=???) at /path/to/foo.rs
+    #10     000011 as b (v=???) at /path/to/foo.rs
+    #9      000010 as a (v=???) at /path/to/foo.rs
+    #8      000012 as main () at /path/to/foo.rs
+    ...
+    ```
+
+[wasmgdb]: https://crates.io/crates/wasmgdb