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/meta/src/lib.rs

@@ -116,6 +116,9 @@ pub fn generate(
             isa::Isa::Arm64 => {
                 // aarch64 doesn't have platform-specific settings.
             }
+            isa::Isa::S390x => {
+                // s390x doesn't have platform-specific settings.
+            }
             isa::Isa::Arm32 | isa::Isa::Riscv => todo!(),
         }
     }