Make CANNOT_DUMP work better on GNU/Linux

Make CANNOT_DUMP work better on GNU/Linux

Clean up some of the bitrot affecting the CANNOT_DUMP code.  This
lets the build succeed again, and fixes the testing framework so
that most test cases now pass.  About twenty test cases still
fail, though, and we still have Bug#24974.
* configure.ac (CANNOT_DUMP): Now empty if CANNOT_DUMP.
(SYSTEM_MALLOC): Now true if CANNOT_DUMP.  There should no longer
be any point to messing with a private memory allocator unless
Emacs is dumping.
* src/alloc.c (alloc_unexec_pre, alloc_unexec_post, check_pure_size):
* src/image.c (reset_image_types):
* src/lastfile.c (my_endbss, _my_endbss, my_endbss_static):
Do not define if CANNOT_DUMP.
* src/emacs.c (might_dump) [CANNOT_DUMP]: Now always false and local.
(daemon_pipe) [!WINDOWSNT]: Now static.
* test/Makefile.in (mostlyclean): Remove *.tmp files.
(make-test-deps.mk): Elide CANNOT_DUMP chatter.