tile: expect new initramfs name from hypervisor file system

The current Tilera boot infrastructure now provides the initramfs
to Linux as a Tilera-hypervisor file named "initramfs", rather than
"initramfs.cpio.gz", as before.  (This makes it reasonable to use
other compression techniques than gzip on the file without having to
worry about the name causing confusion.)  Adapt to use the new name,
but also fall back to checking for the old name.

Cc'ing to stable so that older kernels will remain compatible with
newer Tilera boot infrastructure.

Signed-off-by: Chris Metcalf <cmetcalf@tilera.com>
Cc: stable@vger.kernel.org