Fix clone (CLONE_VM) pid/tid reset (BZ#19957)

As discussed in libc-alpha [1] current clone with CLONE_VM (without
CLONE_THREAD set) will reset the pthread pid/tid fields to -1.  The
issue is since memory is shared between the parent and chield it will
clobber parent's cached pid/tid leading to internal inconsistencies
if the value is not restore.

And even it is restore it may lead to racy condition when between
set/restore a thread might invoke pthread function that validate the
pthread with INVALID_TD_P/INVALID_NOT_TERMINATED_TD_P and thus get
wrong results due inconsistent internal pthread state.

As stated in BZ19957, previously reports of this behaviour was close
with EWONTFIX due the fact usage of clone outside glibc is tricky
since glibc requires consistent internal pthread, while using clone
directly may not provide it. However since now posix_spawn uses
clone(CLONE_VM) to fixes various issues related to previous vfork
usage this issue requires fixing.

The vfork implementation also does something similar, but instead
it negates and restores only the *pid* field and functions that
might access its value know to handle such case (getpid, raise
and pthread ones that uses INVALID_TD_P/INVALID_NOT_TERMINATED_TD_P
macros that check only *tid* field).  Also vfork does not call
__clone directly, instead calling either __NR_vfork or __NR_clone
directly.

So this patch removes this clone behavior by avoiding setting
the pthread pid/tid field for CLONE_VM as well for CLONE_THREAD.
Instead of current approach of:

   int clone(int (*fn)(void *), void *child_stack, int flags, ...)
      [...]
      if (flags & CLONE_THREAD)
        goto do_syscall;
      pid_t new_value;
      if (flags & CLONE_VM)
        new_value = -1;
      else
        new_value = getpid ();
      THREAD_SETMEM (THREAD_SELF, pid, new_value);
      THREAD_SETMEM (THREAD_SELF, tid, new_value);

    do_syscall:
      [...]

The new approach uses:

   int clone(int (*fn)(void *), void *child_stack, int flags, ...)
      [...]
      if (flags & CLONE_THREAD || flags & CLONE_VM)
        goto do_syscall;
      pid_t new_value = getpid ();
      THREAD_SETMEM (THREAD_SELF, pid, new_value);
      THREAD_SETMEM (THREAD_SELF, tid, new_value);

    do_syscall:
      [...]

It also removes the linux tst-getpid2.c test which expects the previous
behavior.

I only fixed the clone.S implementations for i386, x86_64, powerpc,
aarch64, and arm (basically the ones I have a way to actually test it).
If would like to ask for some help on fixing the remaning ones:
sparc (32 and 64-bit), alpha, s390 (32 and 64-bit), microblaze,
sh, ia64, tile, hppa, m68k, nio2, and mips.

Tested on x86_64, i686, x32, powerpc64le, aarch64, and armhf.

[1] https://sourceware.org/ml/libc-alpha/2016-04/msg00307.html
---
 ChangeLog                                         | 13 +++++++++++++
 sysdeps/unix/sysv/linux/Makefile                  |  2 +-
 sysdeps/unix/sysv/linux/aarch64/clone.S           |  8 +++++---
 sysdeps/unix/sysv/linux/arm/clone.S               |  7 +++----
 sysdeps/unix/sysv/linux/i386/clone.S              |  5 +----
 sysdeps/unix/sysv/linux/powerpc/powerpc32/clone.S | 12 +++++++-----
 sysdeps/unix/sysv/linux/powerpc/powerpc64/clone.S | 10 ++++++----
 sysdeps/unix/sysv/linux/tst-getpid2.c             |  2 --
 sysdeps/unix/sysv/linux/x86_64/clone.S            |  7 ++-----
 9 files changed, 38 insertions(+), 28 deletions(-)
 delete mode 100644 sysdeps/unix/sysv/linux/tst-getpid2.c

Fix clone (CLONE_VM) pid/tid reset (BZ#19957)

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