trivial: use core WFI rather than example specific wfi

[karlp]

examples/stm32/l1/stm32l-discovery/button-irq-printf-lowpower/main.c uses it's own __WFI() cmsis style operation, instead of the one in cm3.h... double check it works and replace it

[BuFran]

Isn't the __WFI() macro an compiler builtin ? Double underscores are reserved for compiler and __NOP() exists in the compiler :-)