This adds support for the IBM z/Architecture (s390x-ibm-linux).
The status of the s390x backend in its current form is:
- Wasmtime is fully functional and passes all tests on s390x.
- All back-end features supported, with the exception of SIMD.
- There is still a lot of potential for performance improvements.
- Currently the only supported processor type is z15.
Before this patch, running the x64 new backend would require both
compiling with --features experimental_x64 and running with
`use_new_backend`.
This patches changes this behavior so that the runtime flag is not
needed anymore: using the feature flag will enforce usage of the new
backend everywhere, making using and testing it much simpler:
cargo run --features experimental_x64 ;; other CLI options/flags
This also gives a hint at what the meta language generation would look
like after switching to the new backend.
Compiling only with the x64 codegen flag gives a nice compile time speedup.
