take fdpic-related parts of elf_prstatus out

The only architecture where we might end up using both is arm,
and there we definitely don't want fdpic-related fields in
elf_prstatus - coredump layout of ELF binaries should not
depend upon having the kernel built with the support of ELF_FDPIC
ones.  Just move the fdpic-modified variant into binfmt_elf_fdpic.c
(and call it elf_prstatus_fdpic there)

[name stolen from nico]

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>