__NVIC_EnableIRQ compiler barriers

__NVIC_EnableIRQ compiler barriers

__NVIC_DisableIRQ and __NVIC_EnableIRQ can be used to function as a
mutex-style protection lock against a particular interrupt handler,
similar to __disable_irq and __enable_irq for all interrupts.

However, __NVIC_EnableIRQ, unlike a mutex unlock or __enable_irq, had no
compiler barriers. Being just a volatile write, in the following code
sequence:

    NVIC_DisableIRQ(devx);
    // modify some RAM accessed by devx IRQ handler
    NVIC_EnableIRQ(devx);

there would be nothing preventing the RAM accesses from moved below the
NVIC_EnableIRQ.

Add barriers to NVIC_EnableIRQ, so that the above code works the same as
a mutex or __disable_irq, without any added need to mark the shared RAM
as volatile.

Fixes issue #493