arm, arm926ejs: Fix clear bss loop for zero length bss

This patch fixes the clear bss loop for bss sections that have
zero length, i.e., where __bss_start == __bss_end__.

Signed-off-by: Christian Riesch <christian.riesch@omicron.at>
Cc: Albert Aribaud <albert.u.boot@aribaud.net>