This structure was missed when creating the s390x port.
This is based on the report and patch from William Pitcock, but with a
modified structure defintion to more closely match the kernel's
definition.
Including sys/procfs.h complains unknown type name 'fpreg_t' in
bits/user.h. fpreg_t in bits/signal.h and elf_fpreg_t in bits/user.h
are practically the same.
per_struct is never used, even conflicts with kernel header
asm/ptrace.h