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/x64/lower/isle.rs

@@ -31,6 +31,7 @@ use crate::{
         VCodeConstant, VCodeConstantData,
     },
 };
+use regalloc2::PReg;
 use smallvec::SmallVec;
 use std::boxed::Box;
 use std::convert::TryFrom;
@@ -636,6 +637,16 @@ where
 
         self.gen_call_common(abi, num_rets, caller, args)
     }
+
+    #[inline]
+    fn preg_rbp(&mut self) -> PReg {
+        regs::rbp().to_real_reg().unwrap().into()
+    }
+
+    #[inline]
+    fn preg_rsp(&mut self) -> PReg {
+        regs::rsp().to_real_reg().unwrap().into()
+    }
 }
 
 impl<C> IsleContext<'_, C, Flags, IsaFlags, 6>