arm: Fix to mistake clean the memory space

In currently, when __bss_start is equal to __bss_end__,
The bss loop will clear all the things in memory space.

But just only when __bss_end__ greater than __bss_start__,
we do the clear bss section operation.

Signed-off-by: Zhong Hongbo <bocui107@gmail.com>
Acked-by: Andreas Bießmann <andreas.devel@googlemail.com>