x86_64: ia32 ptrace THREAD_AREA fix

The addr argument to PTRACE_GET_THREAD_AREA and PTRACE_SET_THREAD_AREA is
not a magic constant.  It's derived from the segment register values being
used, which are computed originally from the index used with set_thread_area.
The value does not need to match what a native i386 kernel would accept.
It needs to match the segment selectors that can actually be in use in this
32-bit process.  The 64-bit ptrace support for PTRACE_GET_THREAD_AREA
(normally used only on 32-bit processes) is correct, but the 32-bit emulation
of ptrace is broken.

Signed-off-by: Roland McGrath <roland@redhat.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>