Two Lucet-related fixes to stack overflow handling.

Lucet uses stack probes rather than explicit stack limit checks as
Wasmtime does. In bytecodealliance/lucet#616, I have discovered that I
previously was not running some Lucet runtime tests with the new
backend, so was missing some test failures due to missing pieces in the
new backend.

This PR adds (i) calls to probestack, when enabled, in the prologue of
every function with a stack frame larger than one page (configurable via
flags); and (ii) trap metadata for every instruction on x86-64 that can
access the stack, hence be the first point at which a stack overflow is
detected when the stack pointer is decremented.

cranelift/codegen/src/isa/aarch64/abi.rs

@@ -12,7 +12,7 @@ use crate::{CodegenError, CodegenResult};
 use alloc::boxed::Box;
 use alloc::vec::Vec;
 use regalloc::{RealReg, Reg, RegClass, Set, Writable};
-use smallvec::SmallVec;
+use smallvec::{smallvec, SmallVec};
 
 // We use a generic implementation that factors out AArch64 and x64 ABI commonalities, because
 // these ABIs are very similar.
@@ -508,6 +508,12 @@ impl ABIMachineSpec for AArch64MachineDeps {
         insts
     }
 
+    fn gen_probestack(_: u32) -> SmallVec<[Self::I; 2]> {
+        // TODO: implement if we ever require stack probes on an AArch64 host
+        // (unlikely unless Lucet is ported)
+        smallvec![]
+    }
+
     // Returns stack bytes used as well as instructions. Does not adjust
     // nominal SP offset; abi_impl generic code will do that.
     fn gen_clobber_save(