bit_spinlock: Don't succumb to bit-shift overflow
bit_spinlock: Don't succumb to bit-shift overflow

For bit values larger than would fit in an int, our bit-shift would
overflow in some places and cause undefined behavior.  Use "1ul"
everywhere instead of just "1".

This didn't cause any bugs in practice because our only use of
bit_spinlock is for vmdesc::flags bit 0, and even C can't screw up a
shift by 0.

bowlofstew/sv6,aclements/sv6,aclements/sv6,bowlofstew/sv6,aclements/sv6,bowlofstew/sv6,bowlofstew/sv6,bowlofstew/sv6,aclements/sv6,aclements/sv6