cpu/cortexm_common: remove special cortexm_sleep handle for stm32l152re
cpu/cortexm_common: remove special cortexm_sleep handle for stm32l152re

__set_PRIMASK(state) had been directly inlined to avoid a hardfault that
occured when branching after waking up from sleep with DBG_STANDBY,
DBG_STOP or DBG_SLEEP set in DBG_CR.

The hardfault occured when returning from the branch to irq_restore,
since the function is now inlined the branch does not happen either.

Refer to #14015 for more details.

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