arch: pass cpu option instead of tune option on ARM

Currently, the ARM Config.in logic specifies values for
--with-arch/-march and --with-tune/-mtune, but not for
--with-cpu/-mcpu. However, this causes problems on ARMv4, because
specifying --with-arch=armv4t isn't enough to make gcc generate ARMv4
code: one should also pass --with-cpu=<some ARMv4 CPU>.

Moreover, since Buildroot is generally designed to generate code
specifically for the configured target, it makes sense to give our own
--with-cpu/-mcpu value instead of relying on the default value used by
gcc, and only do small optimizations with -mtune.

Reported-by: Adam Hussein <kryme76@yahoo.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>