factor out setting pc in gdbstub

The code for handling the c and s packets both contain code for setting
the pc.  Move that code out to a common function.

Signed-off-by: Nathan Froyd <froydnj@codesourcery.com>
Acked-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>

git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@7039 c046a42c-6fe2-441c-8c8c-71466251a162