build: Use preferred flag for enabling C++ exceptions in gles2rice.

From the NDK r8d docs (note in particular the last sentence):

LOCAL_CPP_FEATURES
  This is an optional variable that can be defined to indicate
  that your code relies on specific C++ features. To indicate that
  your code uses RTTI (RunTime Type Information), use the following:

      LOCAL_CPP_FEATURES := rtti

  To indicate that your code uses C++ exceptions, use:

      LOCAL_CPP_FEATURES := exceptions

  You can also use both of them with (order is not important):

      LOCAL_CPP_FEATURES := rtti features

  The effect of this variable is to enable the right compiler/linker
  flags when building your modules from sources. For prebuilt binaries,
  this also helps declare which features the binary relies on to ensure
  the final link works correctly.

  It is recommended to use this variable instead of enabling -frtti and
  -fexceptions directly in your LOCAL_CPPFLAGS definition.