Doing the module post processing using some scripting with sed/grep etc
was doable, but performance is of course much better when we do it in C
instead, and it also allows for easier extensibility for additional
postprocessing for e.g. the MODULE_DEVICE_TABLE's.
  
Executing the new helper "modpost" will build the additional C code for all
modules at once, so we can just keep the hash table of exported symbols
in memory instead of rebuilding it for every module.