[bsp][synopsys] add the support of synopsys arc emsk

* the initial support of synopsys designware arc processor
* the initial support of synospsy ARC EM Starter Kit
* the bsp code is based on embarc which is a common SDK for
all synopsys ARC-based boards
* use "scons --gdb" to debug emsk with em9d configuration
* for detailed board information, pls go embarc.org

Signed-off-by: Wayne Ren <wei.ren@synopsys.com>

libcpu/arc/SConscript

@@ -0,0 +1,17 @@
+# RT-Thread building script for bridge
+
+import os
+from building import *
+
+Import('rtconfig')
+
+cwd   = GetCurrentDir()
+group = []
+
+# add common code files
+group = group + SConscript(os.path.join(cwd, 'common', 'SConscript'))
+
+# cpu porting code files
+group = group + SConscript(os.path.join(cwd, rtconfig.CPU, 'SConscript'))
+
+Return('group')