irq_arch.h previously included cpu.h, which in term included the vendor header
files. Those were needed to get the GIE define (general interrupt enable bit).
However, the vendor files use fancy defines like `#define N (0x0004)` that
easily conflict with application code. Due to the widespread use of the IRQ API,
it is better to not include the vendor files in irq_arch.h.
This commit adds a local define for the GIE bit and uses this instead of
including cpu.h.
- Updated to inline-able IRQ API
- Improved robustness of functions
- Added memory barrier to prevent the compiler from moving code outside of
a critical section guarded by irq_disable() ... irq_restore()
- Reduced overhead of `irq_disable()`
- After clearing the global interrupt enable (GIE) bit, IRQs remain enabled
for up to one CPU cycle
- The previous implementation just added a nop to fill that cycle
- This implementation uses the cycle for masking the return value
- Reduced overhead of `irq_restore()`
- Now only one CPU cycle is needed
- `irq_disable()`, `irq_restore()`, and `irq_enable()` work now in constant time
