Merge from origin/emacs-25

2deb20c Port last_marked change to full-program optimizers
27eb668 Adapt GDB scripts to '--enable-check-lisp-object-type' builds
dac64e3 Avoid optimizing out the last_marked[] array

diff --cc etc/emacs-buffer.gdb
Simple merge

diff --cc src/.gdbinit
Simple merge

diff --cc src/alloc.c
Simple merge