meta-agl-distro: Update aarch64 DEFAULTTUNE definition

meta-agl-distro: Update aarch64 DEFAULTTUNE definition

The i.MX8 crosssdk build failure in SPEC-3626 was root caused to the
DEFAULTTUNE variable being over-ridden via a machine over-ride
triggered by a MACHINEOVERRIDES addition in meta-freescale's
imx8mqevk.conf.  To ensure that aarch64 machines use AGL's desired
aarch64 tuning, switch the DEFAULTTUNE definition done in
aarch64-tune.inc to using the "poky-agl" over-ride that comes after
MACHINEOVERRIDES in the OVERRIDES variable definition.

Bug-AGL: SPEC-3626

Signed-off-by: Scott Murray <scott.murray@konsulko.com>
Change-Id: Id07f73056dd75ba7cf130376e65f2731302b7e66
Reviewed-on: https://gerrit.automotivelinux.org/gerrit/c/AGL/meta-agl/+/25488
Tested-by: Jenkins Job builder account <agl-jobbuilder@automotivelinux.org>
ci-image-build: Jenkins Job builder account <agl-jobbuilder@automotivelinux.org>
ci-image-boot-test: Jenkins Job builder account <agl-jobbuilder@automotivelinux.org>
Reviewed-by: Jan-Simon Moeller <jsmoeller@linuxfoundation.org>
(cherry picked from commit 0d89af5ac5d55ca3250688a2ecfcd0f63e02d529)
Reviewed-on: https://gerrit.automotivelinux.org/gerrit/c/AGL/meta-agl/+/25606