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.isle

@@ -3616,3 +3616,15 @@
             (_ Unit (output_builder_push builder ret)))
         (lower_call_rets abi tail builder)))
 
+;;;; Rules for `get_{frame,stack}_pointer` and `get_return_address` ;;;;;;;;;;;;
+
+(rule (lower (get_stack_pointer))
+      (sp))
+
+(rule (lower (get_frame_pointer))
+      (load64 (memarg_stack_off 0 0)))
+
+(rule (lower (get_return_address))
+      ;; The return address is 14 pointer-sized slots above the initial SP. So
+      ;; our offset is `14 * 8 = 112`.
+      (load64 (memarg_initial_sp_offset 112)))