Cranelift: Add instructions for getting the current stack/frame/return pointers (#4573)

* Cranelift: Add instructions for getting the current stack/frame pointers and return address

This is the initial part of https://github.com/bytecodealliance/wasmtime/issues/4535

* x64: Remove `Amode::RbpOffset` and use `Amode::ImmReg` instead

We just special case getting operands from `Amode`s now.

* Fix s390x `get_return_address`; require `preserve_frame_pointers=true`

* Assert that `Amode::ImmRegRegShift` doesn't use rbp/rsp

* Handle non-allocatable registers in Amode::with_allocs

* Use "stack" instead of "r15" on s390x

* r14 is an allocatable register on s390x, so it shouldn't be used with `MovPReg`

cranelift/codegen/meta/src/shared/instructions.rs

@@ -1296,6 +1296,43 @@ pub(crate) fn define(
         .other_side_effects(true),
     );
 
+    ig.push(
+        Inst::new(
+            "get_frame_pointer",
+            r#"
+        Get the address in the frame pointer register.
+
+        Usage of this instruction requires setting `preserve_frame_pointers` to `true`.
+        "#,
+            &formats.nullary,
+        )
+        .operands_out(vec![addr]),
+    );
+
+    ig.push(
+        Inst::new(
+            "get_stack_pointer",
+            r#"
+        Get the address in the stack pointer register.
+        "#,
+            &formats.nullary,
+        )
+        .operands_out(vec![addr]),
+    );
+
+    ig.push(
+        Inst::new(
+            "get_return_address",
+            r#"
+        Get the PC where this function will transfer control to when it returns.
+
+        Usage of this instruction requires setting `preserve_frame_pointers` to `true`.
+        "#,
+            &formats.nullary,
+        )
+        .operands_out(vec![addr]),
+    );
+
     let TableOffset = &TypeVar::new(
         "TableOffset",
         "An unsigned table offset",