Fix memory allocation problems in Cygwin build (Bug#9273).

Fix memory allocation problems in Cygwin build (Bug#9273).

* src/gmalloc.c [CYGWIN] (bss_sbrk_heapbase, bss_sbrk_heapinfo): New
variables.
(malloc_initialize_1) [CYGWIN]: Prepare for reinitializing the
dumped emacs.
(_free_internal_nolock) [CYGWIN]: Ignore requests to free storage
in the static heap.
[CYGWIN] (special_realloc): New function.
(_realloc_internal_nolock) [CYGWIN]: Use the new function on
requests to realloc storage in the static heap.

* src/unexcw.c ( __malloc_initialized): Declare external variable.
(fixup_executable): Force the dumped emacs to reinitialize malloc.