arm-none-eabi target triplet defaults to Little Endian, and so far any
submitted machine code snippets have been verified to be Little Endian.
However a user might override [ARM_]CROSS_COMPILE with an armeb toolchain,
potentially resulting in invalid machine code.

Let's be safe and enforce Little Endian mode for assembler and compiler.

Change-Id: I9cefe24689eaded25d60ffb1f254b254e8d76f9d
Signed-off-by: Andreas Färber <afaerber@suse.de>
Reviewed-on: http://openocd.zylin.com/3498
Tested-by: jenkins
Reviewed-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>