wrap mbgl-core and mbgl-linux in -Wl,--start-group/-Wl,--end-group

Wraps `mbgl-core` and `mbgl-linux` in start-group/end-group linker options to work around cyclic dependencies on Linux.

https://stackoverflow.com/questions/45135/linker-order-gcc/409470#409470

/cc @kkaefer @springmeyer for review