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/filetests/filetests/isa/x64/probestack.clif

@@ -0,0 +1,17 @@
+test compile
+set enable_probestack=true
+target x86_64
+feature "experimental_x64"
+
+function %f1() -> i64 {
+ss0 = explicit_slot 100000
+
+block0:
+  v1 = stack_addr.i64 ss0
+  return v1
+}
+
+; check:  pushq   %rbp
+; nextln:  movq    %rsp, %rbp
+; nextln:  movl    $$100000, %eax
+; nextln:  call    LibCall(Probestack)