gn2bp: Fix compilation of libc++

* Just addition of some required flags to compile libc++.
* rtti is needed for dynamic cast
* -fexceptions is needed to allow libc++ to throw exceptions

This isn't the best way to actually implement this as each and every target that link against libc++ will need to add those 2 flags.

Test: m cronet_aml_components_cronet_android_cronet
Change-Id: I8d76130d860919936bf316c59ded90bb0ded37ee
2 files changed