cleanup: make arch_sigreturn.c files more readable

cleanup: make arch_sigreturn.c files more readable

Make arch_sigreturn.c files more self-sustained.  While they are
still being included by sigreturn.c, the latter no longer defines
arch_sigreturn function.

* linux/alpha/arch_sigreturn.c (arch_sigreturn): Define.
* linux/arm/arch_sigreturn.c: Likewise.
* linux/crisv10/arch_sigreturn.c: Likewise.
* linux/i386/arch_sigreturn.c: Likewise.
* linux/ia64/arch_sigreturn.c: Likewise.
* linux/m68k/arch_sigreturn.c: Likewise.
* linux/microblaze/arch_sigreturn.c: Likewise.
* linux/mips/arch_sigreturn.c: Likewise.
* linux/powerpc/arch_sigreturn.c: Likewise.
* linux/s390/arch_sigreturn.c: Likewise.
* linux/sparc/arch_sigreturn.c: Likewise.
* linux/tile/arch_sigreturn.c: Likewise.
* linux/x32/arch_sigreturn.c: Remove code.
Include "x86_64/arch_sigreturn.c".
* linux/x86_64/arch_sigreturn.c: Stop including "x32/arch_sigreturn.c".
Include "i386/arch_sigreturn.c" with arch_sigreturn temporarily defined
to i386_arch_sigreturn.
(arch_sigreturn): Define.  Add x32 personality support there.
* sigreturn.c: Remove arch_sigreturn header and footer.

Requested-by: Denys Vlasenko <dvlasenk@redhat.com>