Implement stack limit checks for AArch64 (#1573)

This commit implements the stack limit checks in cranelift for the
AArch64 backend. This gets the `stack_limit` argument purpose as well as
a function's global `stack_limit` directive working for the AArch64
backend. I've tested this locally on some hardware and in an emulator
and it looks to be working for basic tests, but I've never really done
AArch64 before so some scrutiny on the instructions would be most
welcome!

cranelift/codegen/src/isa/aarch64/inst/mod.rs

@@ -66,6 +66,8 @@ pub enum ALUOp {
     SubS32,
     /// Sub, setting flags
     SubS64,
+    /// Sub, setting flags, using extended registers
+    SubS64XR,
     /// Multiply-add
     MAdd32,
     /// Multiply-add
@@ -1931,6 +1933,7 @@ impl ShowWithRRU for Inst {
                 ALUOp::AddS64 => ("adds", InstSize::Size64),
                 ALUOp::SubS32 => ("subs", InstSize::Size32),
                 ALUOp::SubS64 => ("subs", InstSize::Size64),
+                ALUOp::SubS64XR => ("subs", InstSize::Size64),
                 ALUOp::MAdd32 => ("madd", InstSize::Size32),
                 ALUOp::MAdd64 => ("madd", InstSize::Size64),
                 ALUOp::MSub32 => ("msub", InstSize::Size32),