Report architecture-specific signal information for core files.

Report architecture-specific signal information for core files.

When opening a core file, if the process terminated due to a signal,
invoke the gdbarch report_signal_info hook to report
architecture-specific information about the signal.

gdb/ChangeLog:

	* corelow.c (core_target_open): Invoke gdbarch report_signal_info
	hook if present.