Add data and instruction synchronization barriers to prevent
interrupts from occurring after NVIC_DisableIRQ is called. This
is a backport of changes made in CMSIS_5.
The current version of cmsis shipping with mbed does not provide the __IOM and friends macros that became standard with CMSIS 4.2+. Patching these into CMSIS to allow vendors to continue updating their support libraries, awaiting a full CMSIS update in mbed (which is past due now).