s390: fix ptrace of user area if the inferior uses vector registers

The floating point registers of a process that uses vector instruction are
not store into task->thread.fp_regs anymore but in the upper halves of the
first 16 vector registers.
The ptrace interface for the peeks and pokes to the user area fails to take
this into account. Fix __peek_user[_compat] and __poke_user[_compat]
to use the vector array for the floating pointer register if the process
has one.

Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>