Pass SIGLIBRT directly to child processes.

Pass SIGLIBRT directly to child processes.

FreeBSD's librt uses SIGLIBRT as an internal signal to implement
SIGEV_THREAD sigevent notifications.  Similar to SIGLWP or SIGCANCEL
this signal should be passed through to child processes by default.

include/ChangeLog:

	* signals.def: Add GDB_SIGNAL_LIBRT.

gdb/ChangeLog:

	* common/signals.c (gdb_signal_from_host): Handle SIGLIBRT.
	(do_gdb_signal_to_host): Likewise.
	* infrun.c (_initialize_infrun): Pass GDB_SIGNAL_LIBRT through to
	programs.
	* proc-events.c (signal_table): Add entry for SIGLIBRT.