mk: fix build on arm64

mk: fix build on arm64

OcteonTx was disabled for causing an internal compiler error on old gcc
versions.
See commit 4f760550a093 ("mk: disable OcteonTx for buggy compilers").

The condition that was added later to apply disabling OcteonTx
only on arm64 caused the condition to never be obeyed because it
compares ["arm64"] to [arm64].

This fixes the condition by using RTE_ARCH instead of CONFIG_RTE_ARCH,
because the former has the quotes removed, while the later doesn't.

Fixes: f3af3e44a444 ("mk: disable OcteonTx for buggy compilers only on arm64")
Cc: stable@dpdk.org
Signed-off-by: Ali Alnubani <alialnu@mellanox.com>
Acked-by: Thomas Monjalon <thomas@monjalon.net>
Acked-by: Jerin Jacob <jerinj@marvell.com>