i want to a very easy way of setting this bit
Pendsv is something that came in with Cortex-M. ARM7 doesn't do any of the system type stuff in Cortex-M. However one can accomplish the effect by having a bit in some mask to raise an IRQ interrupt and unsetting the bit in the interrupt routine. So the IRQ interrupt handler has to test for the various interrupt types and call whatever routine you wanted called when user mode was reached again.