pthread: initialize earlier

boost's asio.hpp module init() function was calling pthread_key_create which
tried to manipulate some libc/pthread.cc unitialized objects.

pthread_key_create then returned -ENOMEM so an exception was thrown by boost
causing the loader's premain() function to fail and the boot to loop.

This patch lower the priority of the initialization of the libc/pthread.cc
objects so they are ready when premain is calling the boost module init()
function.

Signed-off-by: Benoit Canet <benoit@irqsave.net>
Signed-off-by: Avi Kivity <avi@cloudius-systems.com>
prio: remove rendundant VMA_LIST_INIT_PRIO

Signed-off-by: Benoit Canet <benoit@irqsave.net>
Signed-off-by: Avi Kivity <avi@cloudius-systems.com>