Added --multibyte_chars option

Added --multibyte_chars option for ARM and uARM build.

This enables to use multi-byte character such as Japanese or Chinese
especially for string literal.
It should not be affected single-byte character set.  Tested with
Japanese Windows environment.

workspace_tools/toolchains/arm.py

@@ -44,7 +44,7 @@ class ARM(mbedToolchain):
         common = ["-c",
             "--cpu=%s" % cpu, "--gnu",
             "-Otime", "--split_sections", "--apcs=interwork",
-            "--brief_diagnostics", "--restrict"
+            "--brief_diagnostics", "--restrict", "--multibyte_chars"
         ]
 
         if "save-asm" in self.options: