aarch64: GCC10: Add -mno-outline-atomics

GCC10 introduced by default calls to out-of-line helpers to implement
atomic operations with the '-moutline-atomic' option. This is breaking
several tests because the embedded calls are trying to access the
zephyr_data region from userspace that is declared as MT_P_RW_U_NA,
triggering a memory fault.

Since there is currently no support for MT_P_RW_U_RO (and probably never
will be), disable the out-of-line helpers disabling the GCC option.

Signed-off-by: Carlo Caione <ccaione@baylibre.com>