Support: mapped_file_region: Pass MAP_NORESERVE to mmap

This allows mapping larger files, delaying OOM failures until too many
pages of them are accessed.  This is makes the behavior of the
mapped_file_region in this regard consistent between its "Unix" and
"Windows" implementations.

Guard the code witih #if defined(MAP_NORESERVE), consistent with other
uses of MAP_NORESERVE in llvm-project, because some FreeBSD versions do
not provide this flag.

Reviewed By: clayborg

Differential Revision: https://reviews.llvm.org/D96626