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/aarch64/lower/isle.rs

@@ -26,6 +26,7 @@ use crate::{
     isa::unwind::UnwindInst,
     machinst::{ty_bits, InsnOutput, LowerCtx, VCodeConstant, VCodeConstantData},
 };
+use regalloc2::PReg;
 use std::boxed::Box;
 use std::convert::TryFrom;
 use std::vec::Vec;
@@ -466,4 +467,16 @@ where
 
         rd.to_reg()
     }
+
+    fn preg_sp(&mut self) -> PReg {
+        super::regs::stack_reg().to_real_reg().unwrap().into()
+    }
+
+    fn preg_fp(&mut self) -> PReg {
+        super::regs::fp_reg().to_real_reg().unwrap().into()
+    }
+
+    fn preg_link(&mut self) -> PReg {
+        super::regs::link_reg().to_real_reg().unwrap().into()
+    }
 }