Introduce <elf-initfini.h> and ELF_INITFINI for all architectures

This supersedes the init_array sysdeps directory.  It allows us to
check for ELF_INITFINI in both C and assembler code, and skip DT_INIT
and DT_FINI processing completely on newer architectures.

A new header file is needed because <dl-machine.h> is incompatible
with assembler code.  <sysdep.h> is compatible with assembler code,
but it cannot be included in all assembler files because on some
architectures, it redefines register names, and some assembler files
conflict with that.

<elf-initfini.h> is replicated for legacy architectures which need
DT_INIT/DT_FINI support.  New architectures follow the generic default
and disable it.