[roll] Roll fuchsia [kernel] Make sys_system_mexec() size the ZBI properly

Previously, after having coalesced the kernel_vmo into physical memory,
sys_system_mexec() would attempt to create a zbitl::View of the kernel_vmo
in order to interpret and parse the contents as a ZBI.  The strategy for
constructing this view was to use the function
zbitl::StorageFromRawHeader(), which trusts that the metadata of the ZBI
accurately reflect the provenance of the pointer passed as an argument.

However, since the caller has full control over the contents of the
kernel_vmo, they could feign a ZBI much larger than the actual size of
the vmo.  This would lead to sys_system_mexec() using a zbitl::View with
a larger size than has actually been allocated for the kernel_vmo.  Then
when it tries to iterate through the items of said view with a call to
zbitl::CheckBootable(), the iterator would dereference unallocated
memory, causing undefined behavior.

Since vmo objects track their own size, we don't need to depend on
zbitl::StorageFromRawHeader() to create the zbitl::View.  This CL
changes sys_system_mexec() to construct the zbitl::View from a span of
bytes.  This way, even if the contents of the ZBI purport a different
total length, the zbitl::View::end() will accurately reflect the length
of the allocation, preventing iterators from dereferencing out of
bounds.

mcgrathr@ provided the idea to use spans in the construction of the
zbitl::View

PAIR=mcgrathr@

Original-Original-Bug: 512234306
Original-Original-Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/1617468
Original-Original-Revision: 93221231e2e91c531454340f9a7f0f9c8407b4ce
GitOrigin-RevId: 1df29038ac7e34273439cc4ea7b8605f902b7aed
Change-Id: Ic93719918d92eb4faa0de1b2fbbaac2ac41ea49a
Reviewed-on: https://fuchsia-review.googlesource.com/c/integration/+/1640098
Cr-Commit-Position: refs/heads/main@{#194158}