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/s390x/settings.rs

@@ -0,0 +1,9 @@
+//! S390X Settings.
+
+use crate::settings::{self, detail, Builder, Value};
+use core::fmt;
+
+// Include code generated by `cranelift-codegen/meta/src/gen_settings.rs:`. This file contains a
+// public `Flags` struct with an impl for all of the settings defined in
+// `cranelift-codegen/meta/src/isa/s390x/settings.rs`.
+include!(concat!(env!("OUT_DIR"), "/settings-s390x.rs"));