* gcore.c: Reorder include files in alphabetical order. Include

* gcore.c: Reorder include files in alphabetical order.  Include
"gdb_assert.h".  Various coding style fixes.
(derive_stack_segment, derive_heap_segment): Replace check for
non-null BOTTOM and TOP with gdb_assert.
(derive_heap_segment): Replace check for successful creation of
ZERO with gdb_assert.
(make_mem_sec): Use bfd_section_lma to set OSEC->lma.