gdbserver: use current_process in ps_getpid

gdbserver: use current_process in ps_getpid

The following patch ("gdbserver: switch to right process in
find_one_thread") makes it so find_one_thread calls into libthread_db
with a current process but no current thread.  This tripped on ps_getpid
using current_thread in order to get the process' pid.  Get the pid from
`current_process ()` instead, which removes the need to have a current
thread.  Eventually, it would be good to get it from the
gdb_ps_prochandle_t structure, to avoid the need for a current process
as well.

Reviewed-By: Andrew Burgess <aburgess@redhat.com>
Change-Id: I9d2fae266419199a2fbc2fde0a5104c6e0dbd2d5