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/machinst/isle.rs

@@ -879,6 +879,16 @@ macro_rules! isle_prelude_methods {
         fn sink_inst(&mut self, inst: Inst) {
             self.lower_ctx.sink_inst(inst);
         }
+
+        #[inline]
+        fn mem_flags_trusted(&mut self) -> MemFlags {
+            MemFlags::trusted()
+        }
+
+        #[inline]
+        fn preg_to_reg(&mut self, preg: PReg) -> Reg {
+            preg.into()
+        }
     };
 }