sh: Fix ptrace fpu state initialisation

Commit 0ea820cf introduced the PTRACE_GETFPREGS/SETFPREGS cmds,
but gdb-server still accesses the FPU state using the
PTRACE_PEEKUSR/POKEUSR commands. In this case, xstate was not
initialised.

Signed-off-by: Phil Edworthy <phil.edworthy@renesas.com>
Cc: stable@kernel.org
Signed-off-by: Paul Mundt <lethal@linux-sh.org>