Link postgres from all object files at once, to avoid the error-prone

Link postgres from all object files at once, to avoid the error-prone
SUBSYS.o step and allow for better optimization by the linker.

Instead of partial linking into SUBSYS.o, the list of object files is
assembled in objfiles.txt files that are expanded when the final
linking is done.

Because we are not yet sure how long command lines different platforms
can handle, the old way of linking is still available, by defining the
make variable PARTIAL_LINKING (e.g., make all PARTIAL_LINKING=1).  If
we determine that this is necessary for some platforms, then we will
document this in a more prominent place.