Do not redundantly dump constant forwarding objects

Do not redundantly dump constant forwarding objects

* src/pdumper.c (dump_fwd_int, dump_fwd_bool, dump_fwd_obj): Do
not redundantly dump constant forwarding descriptors; restrict
to dumping the objects being forwarded to.
(dump_fwd_buffer_obj): Copy from the dump file into bss rather
than load buffer forwarding descriptors from the dump file
itself.
(dump_fwd_kboard_obj): Delete function.
(dump_fwd): Don't return offset of dumped objects.
(dump_blv): Adjust correspondingly.
(dump_pre_dump_symbol): Improve documentation.  Record offset of
forwarding objects in relation to `emacs_basis' rather than the
dump file.
(dump_symbol): Restore forwarding descriptors to their original
values as static variables in Emacs.  This reduces the size of
dump files by an insignificant 2kb but facilitates certain kinds
of watchpoints on platforms where ASLR cannot be disabled, e.g.,
Android.

(cherry picked from commit 2b7a72b1177a057813f40aacc24fd0cd71f5903a)