Enable thumb on AT91?

[Sean Anderson]

Hi Eugen,

I noticed that 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 git with older GCCs. I looked at the
datasheet for that processor, and noticed that it has thumb support.
Have you considered enabling SYS_THUMB_BUILD? This shrinks SPL by around
30%. I don't have a board to test with, so I don't know if there are any
technical reasons blocking this.

--Sean