mbed OS can't sleep when uVisor is enabled

The RTOS idle thread runs unprivileged. It'd be nice if it, through a register level gateway or vSleep API, could put the system to sleep.

Currently, the RTOS idle thread calls mbed OS sleep, which then calls `hal_sleep`, which has hundreds of implementations that do any number of things that unprivileged code can't do (leading to halts). For example, writing to the `SCB->SCR` register is a nono for unprivileged code (without the register level gateway), yet many implementations do this to control how deeply they want to sleep at various points throughout the program's execution.

Start by tracing the RTOS idle thread through mbed OS sleep to hal_sleep, then inspect how uVisor restricts access to SCB->SCR and related sleep operations. Compare the proposed register-level gateway and vSleep API approaches across the hal_sleep implementations; done means the idle thread can enter the intended sleep mode while uVisor remains enabled without halting.

Written by the indexing model from the issue text.