Port to FreeBSD 11 AMD

Port to FreeBSD 11 AMD

Problem reported by Ashish Shukla (Bug#24892).  I tested
this on FreeBSD 11 x86-64 with HAVE_SBRK manually undefined.
* configure.ac (system_malloc): Set to 'yes' if there is no sbrk.
(sbrk): Check whether it exists.
* src/alloc.c (my_heap_start) [!GNU_LINUX]:
Do not define, since this function is now used only on GNU/Linux,
and sbrk might not exist on other platforms.
(malloc_initialize_hook) [!GNU_LINUX]:
Do not call my_heap_start, since its side effect will never be used.
(Fmemory_limit) [!HAVE_SBRK]: Do not call sbrk.
* src/unexelf.c (unexec) [!HAVE_SBRK]: Assume that nothing like
sbrk exists.