initialize FPU before starting a new thread

Currently new thread inherits FPU state from a previously running thread.
If the running process was rescheduled voluntarily FPU is in more or
less sane state due to x86 calling convention guaranties, but if it was
rescheduled from an interrupt or a fault FPU state may not be clean.

Signed-off-by: Gleb Natapov <gleb@cloudius-systems.com>
Signed-off-by: Avi Kivity <avi@cloudius-systems.com>