Add AArch64 tests to CI (#1526)

* Add AArch64 tests to CI

This commit enhances our CI with an AArch64 builder. Currently we have
no physical hardware to run on so for now we run all tests in an
emulator. The AArch64 build is cross-compiled from x86_64 from Linux.
Tests all happen in release mode with a recent version of QEMU (recent
version because it's so much faster, and in release mode because debug
mode tests take quite a long time in an emulator).

The goal here was not to get all tests passing on CI, but rather to get
AArch64 running on CI and get it green at the same time. To achieve that
goal many tests are now ignored on aarch64 platforms. Many tests fail
due to unimplemented functionality in the aarch64 backend (#1521), and
all wasmtime tests involving compilation are also disabled due to
panicking attempting to generate generate instruction offset information
for trap symbolication (#1523).

Despite this, though, all Cranelift tests and other wasmtime tests
should be runnin on AArch64 through QEMU with this PR. Additionally
we'll have an AArch64 binary release of Wasmtime for Linux, although it
won't be too useful just yet since it will panic on almost all wasm
modules.

* Review comments

tests/all/custom_signal_handler.rs

@@ -87,6 +87,7 @@ mod tests {
     }
 
     #[test]
+    #[cfg_attr(target_arch = "aarch64", ignore)] // FIXME(#1521)
     fn test_custom_signal_handler_single_instance() -> Result<()> {
         let engine = Engine::new(&Config::default());
         let store = Store::new(&engine);