runtime: ensure free and unscavenged spans may be backed by huge pages
This change adds a new sysHugePage function to provide the equivalent of
Linux's madvise(MADV_HUGEPAGE) support to the runtime. It then uses
sysHugePage to mark a newly-coalesced free span as backable by huge
pages to make the freeHugePages approximation a bit more accurate.
The problem being solved here is that if a large free span is composed
of many small spans which were coalesced together, then there's a chance
that they have had madvise(MADV_NOHUGEPAGE) called on them at some point,
which makes freeHugePages less accurate.