GC_malloc_[atomic_]uncollectable generalization

GC_malloc_[atomic_]uncollectable generalization

* include/gc_mark.h (GC_generic_malloc_uncollectable): New public
function declaration.
* malloc.c (GC_generic_malloc_uncollectable): New function
(imlementation mostly copied from GC_malloc_uncollectable).
* malloc.c (GC_malloc_uncollectable, GC_malloc_atomic_uncollectable,
calloc): Use GC_malloc_atomic_uncollectable.
* mallocx.c (GC_generic_or_special_malloc): Likewise.
* mallocx.c (GC_malloc_atomic_uncollectable): Move to malloc.c.