Stack overflow checking with stack probes.

This adds a libcall name, a calling convention, and settings for
emitting stack probes, and implements them for x86 system_v ABIs.

cranelift/filetests/isa/x86/probestack-adjusts-sp.cton

@@ -0,0 +1,29 @@
+test compile
+set is_64bit=1
+set colocated_libcalls=1
+set probestack_func_adjusts_sp=1
+isa x86
+
+; Like %big in probestack.cton, but with the probestack function adjusting
+; the stack pointer itself.
+
+function %big() system_v {
+    ss0 = explicit_slot 300000
+ebb0:
+    return
+}
+; check: function %big(i64 fp [%rbp]) -> i64 fp [%rbp] system_v {
+; nextln:    ss0 = explicit_slot 300000, offset -300016
+; nextln:    ss1 = incoming_arg 16, offset -16
+; nextln:    sig0 = (i64 [%rax]) probestack
+; nextln:    fn0 = colocated %Probestack sig0
+; nextln: 
+; nextln:                                 ebb0(v0: i64 [%rbp]):
+; nextln: [RexOp1pushq#50]                    x86_push v0
+; nextln: [RexOp1copysp#8089]                 copy_special %rsp -> %rbp
+; nextln: [RexOp1pu_id#b8,%rax]               v1 = iconst.i64 0x0004_93e0
+; nextln: [Op1call_id#e8]                     call fn0(v1)
+; nextln: [RexOp1adjustsp_id#8081]            adjust_sp_up_imm 0x0004_93e0
+; nextln: [RexOp1popq#58,%rbp]                v2 = x86_pop.i64
+; nextln: [Op1ret#c3]                         return v2
+; nextln: }