process.c: split struct_user_offsets into architecture-specific include files

process.c: split struct_user_offsets into architecture-specific include files

* Makefile.am (EXTRA_DIST): Add linux/alpha/userent.h,
linux/arm/userent.h, linux/avr32/userent.h, linux/bfin/userent.h,
linux/crisv10/userent.h, linux/crisv32/userent.h,
linux/i386/userent.h, linux/i386/userent0.h, linux/ia64/userent.h,
linux/m68k/userent.h, linux/microblaze/userent.h,
linux/mips/userent.h, linux/or1k/userent.h, linux/powerpc/userent.h,
linux/s390/userent.h, linux/s390/userent0.h, linux/s390/userent1.h,
linux/s390x/userent.h, linux/sh/userent.h, linux/sh/userent0.h,
linux/sh64/userent.h, linux/sparc/userent.h, linux/sparc64/userent.h,
linux/tile/userent.h, linux/userent.h, linux/userent0.h,
linux/x32/userent.h, linux/x86_64/userent.h, and
linux/xtensa/userent.h.
* process.c (struct_user_offsets): Split into architecture-specific
include files, inculde userent.h.