smp: move sub-command "smp_gdb" in file smp.c

smp: move sub-command "smp_gdb" in file smp.c

The code for this command is currently replicated in cortex_a and
in mips_m4k and is going to be added again for cortex_m.
Plus, it is going to be obsoleted by the HW thread pseudo RTOS.

Consolidate in smp.c a single instance of the command. This will
simplify both obsoleting it and adding it to cortex_m, whatever
change comes first.

Change-Id: I03cd857e21fa3f7202fdcee36bcbd5aae30a609d
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/4991
Tested-by: jenkins
Reviewed-by: Tarek BOCHKATI <tarek.bouchkati@gmail.com>
Reviewed-by: Matthias Welwarsky <matthias@welwarsky.de>