Fix the unexec build on GNU/Linux

Fix the unexec build on GNU/Linux

The unexec build on GNU/Linux must use HYBRID_MALLOC (gmalloc.c) and
sheap.c.  This was inadvertently disabled because a configure-time
test for 'sbrk' was moved as side effect of an unrelated change.

* configure.ac: Test for 'sbrk' before using the result in the
decision about SYSTEM_MALLOC and HYBRID_MALLOC.
(HYBRID_MALLOC): Prevent Gnulib from redirecting 'free' to its
replacement 'rpl_free'.

* lib/Makefile.in (not_emacs_OBJECTS): Add mallooc/%.o and free.o.