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/filetests/filetests/isa/aarch64/fp_sp_pc.clif

@@ -0,0 +1,43 @@
+test compile precise-output
+set preserve_frame_pointers=true
+target aarch64
+
+function %fp() -> i64 {
+block0:
+    v0 = get_frame_pointer.i64
+    return v0
+}
+
+;   stp fp, lr, [sp, #-16]!
+;   mov fp, sp
+; block0:
+;   mov x0, fp
+;   ldp fp, lr, [sp], #16
+;   ret
+
+function %sp() -> i64 {
+block0:
+    v0 = get_stack_pointer.i64
+    return v0
+}
+
+;   stp fp, lr, [sp, #-16]!
+;   mov fp, sp
+; block0:
+;   mov x0, sp
+;   ldp fp, lr, [sp], #16
+;   ret
+
+function %return_address() -> i64 {
+block0:
+    v0 = get_return_address.i64
+    return v0
+}
+
+;   stp fp, lr, [sp, #-16]!
+;   mov fp, sp
+; block0:
+;   mov x0, lr
+;   ldp fp, lr, [sp], #16
+;   ret
+