Support IBM z/Architecture

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.

cranelift/codegen/src/isa/mod.rs

@@ -91,6 +91,9 @@ mod arm32;
 #[cfg(feature = "arm64")]
 pub(crate) mod aarch64;
 
+#[cfg(feature = "s390x")]
+mod s390x;
+
 pub mod unwind;
 
 mod call_conv;
@@ -160,6 +163,7 @@ pub fn lookup_variant(triple: Triple, variant: BackendVariant) -> Result<Builder
         }
         (Architecture::Arm { .. }, _) => isa_builder!(arm32, (feature = "arm32"), triple),
         (Architecture::Aarch64 { .. }, _) => isa_builder!(aarch64, (feature = "arm64"), triple),
+        (Architecture::S390x { .. }, _) => isa_builder!(s390x, (feature = "s390x"), triple),
         _ => Err(LookupError::Unsupported),
     }
 }