Fix 342040 Valgrind mishandles clone with CLONE_VFORK | CLONE_VM that clones to a...

Fix 342040 Valgrind mishandles clone with CLONE_VFORK | CLONE_VM that clones to a different stack
Fix 373192 Calling posix_spawn in glibc 2.24 completely broken

Functionally, this patch just does the following 2 changes to the
fork clone handling:
* It does not mask anymore CLONE_VFORK :
  The only effect of this flag is to suspend the parent, waiting for
  the child to either exit or execve.
  If some applications depends on this synchronisation, better keep it,
  as it will not harm to suspend the parent valgrind waiting for the
  child valgrind to exit or execve.
* In case the guest calls the clone syscall providing a non zero client stack,
  set the child guest SP after the syscall, before executing guest instructions.
  Not setting the guest stack ptr was the source of the problem reported
  in the bugs.

This also adds a test case  none/tests/linux/clonev.
Before this patch, test gives a SEGV, which is fixed by the patch.

The patch is however a lot bigger : this fix was touching some (mostly
identical/duplicated) code in all the linux platforms.
So, the clone/fork code has been factorised as much as possible.
This removes about 1700 lines of code.

This has been tested on:
* amd64
* x86
* ppc64 be and le
* ppc32
* arm64

This has been compiled on but *not really tested* on:
* mips64 (not too clear how to properly build and run valgrind on gcc22)

It has *not* been compiled and *not* tested on:
* arm
* mips32
* tilegx
* darwin   (normally, no impact)
* solaris  (normally, no impact)

The changes are relatively mechanical, so it is not impossible that
it will compile and work out of the box on these platforms.
Otherwise, questions welcome.

A few points of interest:
* Some platforms did have a typedef void vki_modify_ldt_t,
  and some platforms had no definition for this type at all.
  To make it easier to factorise, for such platforms, the following has
  been used:
     typedef char vki_modify_ldt_t;
    When the sizeof vki_modify_ldt_t is > 1, then the arg syscall is checked.
  This is somewhat a hack, but was simplifying the factorisation.

* for mips32/mips64 and tilegx, there is a strange unconditional assignment
  of 0 to a register (guest_r2 on mips, guest_r0 on tilegx).
  Unclear what this is, in particular because this is assigned whatever
  the result of the syscall (success or not).

git-svn-id: svn://svn.valgrind.org/valgrind/trunk@16186