In Fuchsia, MmapNoAccess/MmapFixedOrDie are implemented using a global
VMAR, which means that MmapNoAccess can only be called once. This works
for the sanitizer allocator but *not* for the Scudo allocator.
Hence, this changeset introduces a new ReservedAddressRange object to
serve as the new API for these calls. In this changeset, the object
still calls into the old Mmap implementations.
The next changeset two changesets will convert the sanitizer and scudo
allocators to use the new APIs, respectively. (ReservedAddressRange will
replace the SecondaryHeader in Scudo.)
Finally, a last changeset will update the Fuchsia implementation.
Feedback welcome, can provide code pointers to the upcoming changesets
upon request, etc