Default to ARMv7

The current mechanism for determining ARM architecture version
breaks down when cross-compiling. Default to ARMv7 if the user
specifies ARCH=arm. Users wishing to cross-compile for ARMv6
platforms such as the Raspberry Pi should use ARCH=armv6 as
previously required.

Acked-by: Alexander Kartashov <alekskartashov@parallels.com>

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>