[sanitizer][fuchsia] Implement ReleaseMemoryPagesToOS

The `zx_vmar_op_range` allows us to decommit memory pages without
needing a handle to the underlying vmo, as long as we have a handle to
a vmar that contains this mapping. This allows us to implement the
`ReleaseMemoryPagesToOS` function by decommitting the memory using a
handle to the root vmar.

Reviewed By: mcgrathr

Differential Revision: https://reviews.llvm.org/D95384