[PATCH] add dump_stack(): cross-arch backtrace

[PATCH] add dump_stack(): cross-arch backtrace

From Christoph Hellwig, also present in 2.4.

Create an arch-independent `dump_stack()' function.  So we don't need to do

#ifdef CONFIG_X86
	show_stack(0);		/* No prototype in scope! */
#endif

any more.

The whole dump_stack() implementation is delegated to the architecture.
If it doesn't provide one, there is a default do-nothing library
function.