gpu: nova-core: Add a slice-buffer (sbuffer) datastructure

gpu: nova-core: Add a slice-buffer (sbuffer) datastructure

A data structure that can be used to write across multiple slices which
may be out of order in memory. This lets SBuffer user correctly and
safely write out of memory order, without error-prone tracking of
pointers/offsets.

    let mut buf1 = [0u8; 3];
    let mut buf2 = [0u8; 5];
    let mut sbuffer = SBuffer::new([&mut buf1[..], &mut buf2[..]]);

    let data = b"hello";
    let result = sbuffer.write(data);

Reviewed-by: Lyude Paul <lyude@redhat.com>
Co-developed-by: Alistair Popple <apopple@nvidia.com>
Signed-off-by: Alistair Popple <apopple@nvidia.com>
Signed-off-by: Joel Fernandes <joelagnelf@nvidia.com>
Co-developed-by: Alexandre Courbot <acourbot@nvidia.com>
Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
Message-ID: <20251110-gsp_boot-v9-6-8ae4058e3c0e@nvidia.com>