blackfin: kgdb: call generic_exec_single() directly

Current generic API smp_call_function_single() is changed to avoid raise IPI
and call function in IPI handler on the same core which is necessary to support
KGDB switch master core in SMP case, so call generic_exec_single() directly
instead of smp_call_function_single().

Signed-off-by: Sonic Zhang <sonic.zhang@analog.com>
Signed-off-by: Bob Liu <lliubbo@gmail.com>