Internal linkage for gmalloc etc. if HYBRID_MALLOC

Internal linkage for gmalloc etc. if HYBRID_MALLOC

This avoids clashes with symbols if the after-dump malloc is
derived from Doug Lea's implementation (Bug#22086).

* src/gmalloc.c (emacs_abort, __morecore, __default_morecore):
Move declarations up.  For HYBRID_MALLOC, turn all `extern'
declarations below to `static' ones.
(aligned_alloc): Declare for !MSDOS as well.
(heapsize, _fraghead): Move resp. copy declaration downwards.
For HYBRID_MALLOC, conditionalize out the other definitions,
since the previous `static' declarations double as tentative
definitions, anyway.
(_malloc, _free, _realloc, __free_hook, _aligned_blocks)
(__realloc_hook, __memalign_hook): Conditionalize out.
(cfree, memalign, valloc): Ditto.