-Wwrite-strings: Constify target_pid_to_str and target_thread_extra_thread_info

-Wwrite-strings: Constify target_pid_to_str and target_thread_extra_thread_info

-Wwrite-strings flagged a missing cast for example here:

   static char *
   ravenscar_extra_thread_info (struct target_ops *self, struct thread_info *tp)
   {
     return "Ravenscar task";

Since callers are not supposed to free the string returned by these
methods, change the methods' signature to return const strings.

gdb/ChangeLog:
2017-04-05  Pedro Alves  <palves@redhat.com>

	* aix-thread.c (aix_thread_pid_to_str)
	(aix_thread_extra_thread_info): Constify.
	* bsd-kvm.c (bsd_kvm_pid_to_str): Constify.
	* bsd-uthread.c (bsd_uthread_extra_thread_info)
	(bsd_uthread_pid_to_str): Constify.
	* corelow.c (core_pid_to_str): Constify.
	* darwin-nat.c (darwin_pid_to_str): Constify.
	* fbsd-nat.c (fbsd_pid_to_str): Constify.
	* fbsd-tdep.c (fbsd_core_pid_to_str, gdbarch_core_pid_to_str):
	Constify.
	* gnu-nat.c (gnu_pid_to_str): Constify.
	* go32-nat.c (go32_pid_to_str): Constify.
	* i386-cygwin-tdep.c (i386_windows_core_pid_to_str): Constify.
	* inf-ptrace.c (inf_ptrace_pid_to_str): Constify.
	* inferior.c (inferior_pid_to_str): Constify.
	* linux-nat.c (linux_nat_pid_to_str): Constify.
	* linux-tdep.c (linux_core_pid_to_str): Constify.
	* linux-thread-db.c (thread_db_pid_to_str)
	(thread_db_extra_thread_info): Constify.
	* nto-tdep.c (nto_extra_thread_info): Constify.
	* nto-tdep.h (nto_extra_thread_info): Constify.
	* obsd-nat.c (obsd_pid_to_str): Constify.
	* procfs.c (procfs_pid_to_str): Constify.
	* ravenscar-thread.c (ravenscar_extra_thread_info)
	(ravenscar_pid_to_str): Constify.
	* remote-sim.c (gdbsim_pid_to_str): Constify.
	* remote.c (remote_threads_extra_info, remote_pid_to_str):
	Constify.
	* sol-thread.c (solaris_pid_to_str): Constify.
	* sol2-tdep.c (sol2_core_pid_to_str): Constify.
	* sol2-tdep.h (sol2_core_pid_to_str): Constify.
	* target.c (default_pid_to_str, target_pid_to_str)
	(normal_pid_to_str, default_pid_to_str): Constify.
	* target.h (target_ops::to_pid_to_str)
	(target_ops::to_extra_thread_info): Constify.
	(target_pid_to_str, normal_pid_to_str): Constify.
	* windows-nat.c (windows_pid_to_str): Constify.
	* gdbarch.sh (core_pid_to_str): Constify.
	* target-delegates.c: Regenerate.
	* gdbarch.h, gdbarch.c: Regenerate.