nptl: restore .interp section in libpthread.so

In commit 02657da2, .interp section
was removed from libpthread.so.  This led to an error:

  $ /lib64/libpthread.so.0
  Native POSIX Threads Library by Ulrich Drepper et al
  Copyright (C) 2015 Free Software Foundation, Inc.
  This is free software; see the source for copying conditions.
  There is NO warranty; not even for MERCHANTABILITY or FITNESS FOR A
  PARTICULAR PURPOSE.
  Forced unwind support included.
  Segmentation fault

  (gdb) bt
  #0  0x00000000000055a6 in _exit@plt ()

Unfortunately, there is no way to add a regression test for the bug
because .interp specifies the path to dynamic linker of the target
system.

[BZ #18479]
* nptl/pt-interp.c: New file.
* nptl/Makefile (libpthread-routines, libpthread-shared-only-routines):
Add pt-interp.
[$(build-shared) = yes] ($(objpfx)pt-interp.os): Depend on
$(common-objpfx)runtime-linker.h.