make gdb_id() generally avialable and rename it to cpu_index()

The following patch also needs this API, so make it generally avialable.
The function gdb_id() will not be used in gdbstub.c now, so its name is
not suitable, and rename it to cpu_index()

Signed-off-by: Wen Congyang <wency@cn.fujitsu.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>