Fix portSWITCH_TO_USER_MODE() on Armv7-M MPU ports (#803)

Fix portSWITCH_TO_USER_MODE() on Armv7-M MPU ports (#803)

A task's privilege level is stored in ulTaskFlag member in the TCB. Current
implementation of portSWITCH_TO_USER_MODE() does not update this
flag but just lowers the processor's privilege level. This results in many
APIs incorrectly determining task's privilege level and access permissions -

- xPortIsAuthorizedToAccessBuffer
- xPortIsTaskPrivileged
- xPortIsAuthorizedToAccessKernelObject

This PR fixes the portSWITCH_TO_USER_MODE() implementation to correctly
update the ulTaskFlag member in the TCB before lowering the processor's
privilege level.