sigreturn.c: split arch specific code into separate arch files

Split code that use arch-specific registers to separate arch files.

* sigreturn.c (sys_sigreturn): Move arch-specific code
to linux/*/arch_sigreturn.c, include "arch_sigreturn.c".
* linux/arch_sigreturn.c: New file.
* Makefile.am (EXTRA_DIST): Add linux/arch_sigreturn.c
and linux/*/arch_sigreturn.c files.
22 files changed