arm: Enable SYS_THUMB_BUILD on AT91

Several AT91 boards are quite close to their SPL size limit. For example,
sama5d27_wlsom1_ek_mmc is just 173 bytes short of its limit and doesn't
even fit with older GCCs.

All AT91 processors should have thumb support. Enable SYS_THUMB_BUILD. This
shrinks SPL by around 30%.

Signed-off-by: Sean Anderson <seanga2@gmail.com>