[CODEGEN] Use correct math intrin for metal (#562)

src/codegen/intrin_rule_metal.cc

+/*!
+ *  Copyright (c) 2017 by Contributors
+ * \file intrin_rule_metal.cc
+ * \brief Metal intrinsic rules.
+ */
+#include "./intrin_rule.h"
+
+namespace tvm {
+namespace codegen {
+namespace intrin {
+
+TVM_REGISTER_GLOBAL("tvm.intrin.rule.metal.exp")
+.set_body(DispatchExtern<FloatDirect>);
+
+TVM_REGISTER_GLOBAL("tvm.intrin.rule.metal.log")
+.set_body(DispatchExtern<FloatDirect>);
+
+TVM_REGISTER_GLOBAL("tvm.intrin.rule.metal.tanh")
+.set_body(DispatchExtern<FloatDirect>);
+
+TVM_REGISTER_GLOBAL("tvm.intrin.rule.metal.sqrt")
+.set_body(DispatchExtern<FloatDirect>);
+
+TVM_REGISTER_GLOBAL("tvm.intrin.rule.metal.pow")
+.set_body(DispatchExtern<FloatDirect>);
+
+}  // namespace intrin
+}  // namespace codegen
+}  // namespace tvm