[fuchsia] Update Fuchsia with a new mmap implementation.

Summary:
    Now that the sanitizer_common interface for MmapNoAccess / MmapFixed
    have been refactored to allow a more OO-esque access pattern, update the
    Fuchsia mmap implementation to take advantage of this.
    
    Previously MmapNoAccess / MmapFixed relied on a global allocator_vmar,
    since the sanitizer_allocator only called MmapNoAccess once.  Now, we
    create a new VMAR per ReservedAddressRange object.
    
    This allows the sanitizer allocator to work in tandem with the Scudo
    secondary allocator.
    
    This is part 4 of a 4 part changeset:
    * part 1 https://reviews.llvm.org/D38593
    * part 2 https://reviews.llvm.org/D38592
    * part 3 https://reviews.llvm.org/D38593

Reviewers: mcgrathr, cryptoad

Reviewed By: cryptoad

Subscribers: alekseyshl, mcgrathr, kubamracek, mehdi_amini

Differential Revision: https://reviews.llvm.org/D38595

llvm-svn: 319083