Add support for extracting thread names from cores.

Add support for extracting thread names from cores.

Add a new gdbarch method to extract a thread name from a core for a
given thread.  Use this new method in core_thread_name to implement the
to_thread_name target op.

gdb/ChangeLog:

	* corelow.c (core_thread_name): New function.
	(init_core_ops): Use "core_thread_name" for the "to_thread_name"
	target op.
	* gdbarch.sh (core_thread_name): New gdbarch callback.
	* gdbarch.h: Re-generate.
	* gdbarch.c: Re-generate.