exynos: imply SYS_THUMB_BUILD

This patch allows smaller binaries.
This is needed for and has been tested on Arndale board, as u-boot.bin is
now bigger than the 512K load limit, with GCC8, without thumb mode.

Signed-off-by: Guillaume GARDET <guillaume.gardet@free.fr>

Cc: Albert Aribaud <albert.u.boot@aribaud.net>
Cc: Minkyu Kang <mk7.kang@samsung.com>
Cc: Tom Rini <trini@konsulko.com>
Acked-by: Lukasz Majewski <lukma@denx.de>
Acked-by: Minkyu Kang <mk7.kang@samsung.com>
Signed-off-by: Minkyu Kang <mk7.kang@samsung.com>