[sgen-bridge] Introduce accounting mode to bridge processing.

This accounting mode logs the "cost" of each bridge-object during bridge processing.
This should give a very good first clue on how expensive each object is.
[genproj] Some hygiene changes to genproj, to be able to even digest this
Hopefully fix a build error in libgc on some compilers.

../../libgc/alloc.c: In function ‘GC_stopped_mark’:
../../libgc/alloc.c:501:9: error: implicit declaration of function ‘GC_cond_register_dynamic_libraries’ [-Werror=implicit-function-declaration]
         GC_cond_register_dynamic_libraries();