Protobuf uses `dynamic_cast` on polymorphic types which cannot be
inferred properly at compile-time, which can lead to unexpected behavior.
As a code safety measure, enable C++ RTTI for protobuf.
Correct various pre-processor defines for sparc64 and in mozjemalloc use the JS arm64 allocator on Linux/sparc64.
This corrects build problems opn Linux sparc64 and is in line with bugzilla bug #1275204.
