nouveau: Don't require RTTI and use it only when enabled

The only case RTTI is used in nouveau is type assertion at:
File src/gallium/drivers/nouveau/codegen/nv50_ir.cpp:

assert(typeid(*i) == typeid(*this));

This assertion is used 'to be on the safe side' only and not mandatory.

In Android we do not have rtti for libLLVM therefore this assertion
has to be skipped.