remove global variable in_vm86call, set vm86 calling flag in PCB flags.
2.Fix vm86 BIOS calling preempted problem by changing vm86_lock mutex type
from MTX_DEF to MTX_SPIN. vm86pcb is not remembered in thread struct,
when the thread calling vm86 BIOS is preempted by interrupt thread,
and later switching back to the thread would cause incorrect context be
loaded into CPU registers, this leads to kernel crash.
b2a02ec1dc