Enable build on Mac OS X with clang

* Add build target 'darwin'
* Modify compiler options to work with clang
* Support 'jerry-libc' on Mac OS X

For MPU target build, install `gcc-arm-none-eabi` tool chain using `Homebrew` as following.
````
brew tap PX4/homebrew-px4
brew update
brew install gcc-arm-none-eabi
````
https://pixhawk.org/dev/toolchain_installation_mac

JerryScript-DCO-1.0-Signed-off-by: Sung-Jae Lee sjlee@mail.com

build/configs/toolchain_darwin_x86_64.cmake

@@ -0,0 +1,23 @@
+# Copyright 2015 Samsung Electronics Co., Ltd.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+set(CMAKE_SYSTEM_NAME Darwin)
+set(CMAKE_SYSTEM_PROCESSOR x86_64)
+
+find_program(CMAKE_C_COMPILER NAMES cc) 
+find_program(CMAKE_CXX_COMPILER NAMES c++)
+# FIXME: This could break cross compilation, when the strip is not for the target architecture
+find_program(CMAKE_STRIP NAMES strip)
+
+#set(FLAGS_COMMON_ARCH -ffixed-rbp)