systemd: avoid boot failures for mips64

systemd: avoid boot failures for mips64

If we start a systemd based qemumips64 target with 'pam' enabled in
DISTRO_FEATURES, we could not login in successfully. After you provide
username and password to the login prompt, the login prompt just comes up
again. Besides, there are other boot failures.

Append '-O0' flag for mips64 target as a workaround to this problem.

[YOCTO #10250]

Signed-off-by: Chen Qi <Qi.Chen@windriver.com>