stm32: Fix stm32f4x startup

Signed-off-by: Andrew 'Necromant' Andrianov <andrew@ncrmnt.org>
lib: newlib stubs for stm32f4x

Signed-off-by: Andrew 'Necromant' Andrianov <andrew@ncrmnt.org>
stm32: properly enable/disable IRQs in init code

Signed-off-by: Andrew 'Necromant' Andrianov <andrew@ncrmnt.org>