arch/arm: Cortex-M3 provides only Thumb-2

arch/arm: Cortex-M3 provides only Thumb-2

The Cortex-M cores only support Thumb-2, not Thumb. In fact, Thumb-2
is a superset of Thumb, and we could have a single option for both in
Buildroot, since -mthumb on ARMv4/v5 means original Thumb, while
-mthumb on ARMv7 means Thumb 2. However, for clarity, it makes sense
to have two separate options. But in this case, Cortex-M3 should not
advertise that it supports Thumb, as in fact selecting Thumb would
generate Thumb-2 code.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>