ARM64 backend, part 9 / 11: wasmtime support.

This commit adds a few odds and ends required to build wasmtime on ARM64
with the new backend. In particular, it adds:

- Support for the `Arm64Call` relocation type.
- Support for fetching the trap PC when a signal is received.
- A hook for `SIGTRAP`, which is sent by the `brk` opcode (in contrast to
  x86's `SIGILL`).

With the patch sequence up to and including this patch applied,
`wasmtime` can now compile and successfully execute code on arm64. Not
all tests pass yet, but basic Wasm/WASI tests work correctly.

tests/custom_signal_handler.rs

@@ -122,7 +122,7 @@ mod tests {
                 .downcast::<Trap>()?;
             assert!(
                 trap.message()
-                    .starts_with("wasm trap: out of bounds memory access"),
+                    .starts_with("wasm trap: out of bounds"),
                 "bad trap message: {:?}",
                 trap.message()
             );
@@ -149,7 +149,7 @@ mod tests {
                 .downcast::<Trap>()?;
             assert!(trap
                 .message()
-                .starts_with("wasm trap: out of bounds memory access"));
+                .starts_with("wasm trap: out of bounds"));
         }
         Ok(())
     }