gdb: make thread_info::executing private

gdb: make thread_info::executing private

Rename thread_info::executing to thread_info::m_executing, and make it
private.  Add a new get/set member functions, and convert GDB to make
use of these.

The only real change of interest in this patch is in thread.c where I
have deleted the helper function set_executing_thread, and now just
use the new set function thread_info::set_executing.  However, the old
helper function set_executing_thread included some code to reset the
thread's stop_pc, so I moved this code into the new function
thread_info::set_executing.  However, I don't believe there is
anywhere that this results in a change of behaviour, previously the
executing flag was always set true through a call to
set_executing_thread anyway.