[ARM] 2970/1: Use -mtune=arm1136j-s when building for CPU_V6 targets

Patch from George G. Davis

When building for CPU_V6 targets, we should use -mtune=arm1136j-s rather
than -mtune=strongarm but fall back to the later in case someone is
using an older toolchain (although they should really upgrade instead).

Signed-off-by: George G. Davis <gdavis@mvista.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>