am33x-cm3: Enforce correct -march option

am33x-cm3: Enforce correct -march option

Buildroot uses a normal ARM compiler to build the AM33X CM3 firmware
(which runs on a Cortex-M3 processor), but Buildroot will have a
default -march value that doesn't necessarily match the one needed for
Cortex-M3, leading to build failures (gcc complains that the
-mcpu=cortex-m3 option being passed is not compatible with the
selected -march).

Fix this by explicitly indicating -march=armv7-m.

Fixes:

   http://autobuild.buildroot.net/results/db1/db13623b1701bfe678c302e8f88c7473cb2345f9/build-end.log

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>