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-disabled.cton

@@ -0,0 +1,25 @@
+test compile
+set is_64bit=1
+set colocated_libcalls=1
+set probestack_enabled=0
+isa x86
+
+; Like %big in probestack.cton, but with probes disabled.
+
+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: 
+; nextln:                                 ebb0(v0: i64 [%rbp]):
+; nextln: [RexOp1pushq#50]                    x86_push v0
+; nextln: [RexOp1copysp#8089]                 copy_special %rsp -> %rbp
+; nextln: [RexOp1adjustsp_id#d081]            adjust_sp_down_imm 0x0004_93e0
+; nextln: [RexOp1adjustsp_id#8081]            adjust_sp_up_imm 0x0004_93e0
+; nextln: [RexOp1popq#58,%rbp]                v1 = x86_pop.i64
+; nextln: [Op1ret#c3]                         return v1
+; nextln: }