cortex_a_poll: minor code factorization to enhance readability

cortex_a_poll: minor code factorization to enhance readability

cortex_a_debug_entry and update_halt_gdb are called in two consecutive
conditions which are complementary, so externalizing the common code
makes the conditions' body lighter

With the removal of LOG_DEBUG(" ") since it does not look too informative

Change-Id: I0c54e413619576bb3af164f2dcf256c5a862c5fd
Signed-off-by: Tarek BOCHKATI <tarek.bouchkati@gmail.com>
Reviewed-on: http://openocd.zylin.com/4832
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>
Tested-by: jenkins