This a preparatory patch which allows UML to avoid changing linux/linkage.h.

It restructures linkage.h so that all of the arch-specific stuff is in
asm-*/linkage.h.  linux/linkage.h is now arch independent.  It should be
functionally unchanged.

Five arches, i386, ia64, sh, m68k, and arm, have non-empty linkage.h files.
The other arch linkage.h files are all empty.

Also, __ALIGN_STR is no longer defined independently of __ALIGN.  It is now
derived by stringizing __ALIGN.

Following a suggestion by Keith Owens, stringify.h was generalized to allow
commas in its argument, linkage.h now includes it, and no longer defines its
own stringify macros.