Fix GET_MEM argument rounding in GC_scratch_alloc and similar

Fix GET_MEM argument rounding in GC_scratch_alloc and similar
(Prevent abort in GC_unix_mmap_get_mem if the allocation size is not
a multiple of a page size.)

* backgraph.c (new_back_edges, push_in_progress): Use
ROUNDUP_PAGESIZE_IF_MMAP() to adjust GET_MEM() argument (when needed).
* headers.c (GC_scratch_alloc): Likewise.
* misc.c (GC_envfile_init): Likewise.
* include/private/gc_priv.h (ROUNDUP_PAGESIZE_IF_MMAP): New macro.
* include/private/gcconfig.h (MMAP_SUPPORTED): Move definition from
os_dep.c (as needed for ROUNDUP_PAGESIZE_IF_MMAP() definition).
* include/private/gcconfig.h (GET_MEM): Refine comment (regarding its
argument).