cleanup: make get_scno.c files more readable

cleanup: make get_scno.c files more readable

Make get_scno.c files more self-contained.  While they are still
being included by syscall.c, the latter no longer provides
a function framework for them.

* linux/aarch64/get_scno.c (arch_get_scno): Define.
* linux/alpha/get_scno.c: Likewise.
* linux/arc/get_scno.c: Likewise.
* linux/arm/get_scno.c: Likewise.
* linux/avr32/get_scno.c: Likewise.
* linux/bfin/get_scno.c: Likewise.
* linux/crisv10/get_scno.c: Likewise.
* linux/hppa/get_scno.c: Likewise.
* linux/i386/get_scno.c: Likewise.
* linux/ia64/get_scno.c: Likewise.
* linux/m68k/get_scno.c: Likewise.
* linux/metag/get_scno.c: Likewise.
* linux/microblaze/get_scno.c: Likewise.
* linux/mips/get_scno.c: Likewise.
* linux/nios2/get_scno.c: Likewise.
* linux/or1k/get_scno.c: Likewise.
* linux/powerpc/get_scno.c: Likewise.
* linux/powerpc64/get_scno.c: Likewise.
* linux/s390/get_scno.c: Likewise.
* linux/sh/get_scno.c: Likewise.
* linux/sh64/get_scno.c: Likewise.
* linux/sparc/get_scno.c: Likewise.
* linux/sparc64/get_scno.c: Likewise.
* linux/tile/get_scno.c: Likewise.
* linux/x86_64/get_scno.c: Likewise.
* linux/xtensa/get_scno.c: Likewise.
* syscall.c (arch_get_scno): New declaration.
Include "get_scno.c" in file scope.
(get_scno): Use arch_get_scno.

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