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/isa/s390x/mod.rs

@@ -0,0 +1,31 @@
+use crate::cdsl::cpu_modes::CpuMode;
+use crate::cdsl::instructions::{InstructionGroupBuilder, InstructionPredicateMap};
+use crate::cdsl::isa::TargetIsa;
+use crate::cdsl::recipes::Recipes;
+use crate::cdsl::regs::IsaRegsBuilder;
+use crate::cdsl::settings::SettingGroupBuilder;
+
+use crate::shared::Definitions as SharedDefinitions;
+
+pub(crate) fn define(shared_defs: &mut SharedDefinitions) -> TargetIsa {
+    let inst_group = InstructionGroupBuilder::new(&mut shared_defs.all_instructions).build();
+    let settings = SettingGroupBuilder::new("s390x").build();
+    let regs = IsaRegsBuilder::new().build();
+    let recipes = Recipes::new();
+    let encodings_predicates = InstructionPredicateMap::new();
+
+    let mut mode = CpuMode::new("s390x");
+    let expand = shared_defs.transform_groups.by_name("expand");
+    mode.legalize_default(expand);
+    let cpu_modes = vec![mode];
+
+    TargetIsa::new(
+        "s390x",
+        inst_group,
+        settings,
+        regs,
+        recipes,
+        cpu_modes,
+        encodings_predicates,
+    )
+}