compiler: implement recover()

Doesn't do anything useful yet as running deferred calls are not
executed during panic. Only implemented to get code to compile.

compiler/compiler.go

@@ -1787,6 +1787,8 @@ func (c *Compiler) parseBuiltin(frame *Frame, args []ssa.Value, callName string)
 			c.createRuntimeCall("printnl", nil, "")
 		}
 		return llvm.Value{}, nil // print() or println() returns void
+	case "recover":
+		return c.createRuntimeCall("_recover", nil, ""), nil
 	case "ssa:wrapnilchk":
 		// TODO: do an actual nil check?
 		return c.parseExpr(frame, args[0])

src/runtime/panic.go

@@ -15,6 +15,13 @@ func runtimePanic(msg string) {
 	abort()
 }
 
+// Try to recover a panicking goroutine.
+func _recover() interface{} {
+	// Deferred functions are currently not executed during panic, so there is
+	// no way this can return anything besides nil.
+	return nil
+}
+
 // Check for bounds in *ssa.Index, *ssa.IndexAddr and *ssa.Lookup.
 func lookupBoundsCheck(length lenType, index int) {
 	if index < 0 || index >= int(length) {