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/src/isa/s390x/lower.rs

@@ -180,7 +180,10 @@ impl LowerBackend for S390xBackend {
             | Opcode::Return
             | Opcode::StackAddr
             | Opcode::FuncAddr
-            | Opcode::SymbolValue => {
+            | Opcode::SymbolValue
+            | Opcode::GetFramePointer
+            | Opcode::GetStackPointer
+            | Opcode::GetReturnAddress => {
                 unreachable!(
                     "implemented in ISLE: inst = `{}`, type = `{:?}`",
                     ctx.dfg().display_inst(ir_inst),