configure: Drop armv4l/armv4b distinction in $cpu

Drop the distinction between armv4l/armv4b in the $cpu variable
(ie host cpu type) in favour of calling everything 'arm'. This
makes it the same as the ARCH setting and removes some special
casing. The only thing we were using the distinction for was to
decide which endianness to use in cross compilation; do a cpp
define check there instead.

Reviewed-by: Andreas Färber <afaerber@suse.de>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Andrzej Zaborowski <andrew.zaborowski@intel.com>
