Adds external/libcxx and external/libcxxabi to manifest

Change-Id: I520b595d69834ff00140859e1d06dcc6d347e078

default.xml

@@ -147,6 +147,8 @@
   <project path="external/junit" name="platform/external/junit" />
   <project path="external/kernel-headers" name="platform/external/kernel-headers" />
   <project path="external/libcap-ng" name="platform/external/libcap-ng" />
+  <project path="external/libcxx" name="platform/external/libcxx_35a" />
+  <project path="external/libcxxabi" name="platform/external/libcxxabi_35a" />
   <project path="external/libffi" name="platform/external/libffi" />
   <project path="external/libgsm" name="platform/external/libgsm" groups="pdk" />
   <project path="external/liblzf" name="platform/external/liblzf" groups="pdk" />