Switch default to new x86_64 backend.

This PR switches the default backend on x86, for both the
`cranelift-codegen` crate and for Wasmtime, to the new
(`MachInst`-style, `VCode`-based) backend that has been under
development and testing for some time now.

The old backend is still available by default in builds with the
`old-x86-backend` feature, or by requesting `BackendVariant::Legacy`
from the appropriate APIs.

As part of that switch, it adds some more runtime-configurable plumbing
to the testing infrastructure so that tests can be run using the
appropriate backend. `clif-util test` is now capable of parsing a
backend selector option from filetests and instantiating the correct
backend.

CI has been updated so that the old x86 backend continues to run its
tests, just as we used to run the new x64 backend separately.

At some point, we will remove the old x86 backend entirely, once we are
satisfied that the new backend has not caused any unforeseen issues and
we do not need to revert.

fuzz/Cargo.toml

@@ -20,7 +20,10 @@ wasmtime-fuzzing = { path = "../crates/fuzzing" }
 wasm-smith = "0.4.0"
 
 [features]
-experimental_x64 = ["wasmtime-fuzzing/experimental_x64"]
+# Leave a stub feature with no side-effects in place for now: the OSS-Fuzz
+# config builds fuzz targets with this feature enabled and we don't want to
+# break the build.
+experimental_x64 = []
 
 [[bin]]
 name = "compile"