arm: fix -march for ARM11

In GCC 9 support for the Armv5 and Armv5E architectures (which have no
known implementations) has been removed, cf.
https://gcc.gnu.org/gcc-9/changes.html



ARM11 is an armv6 implementation. So change the architecture flag for the
compiler to armv6 for ARM11.

Suggested-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Fabio Estevam <festevam@gmail.com>