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.
008e2d3b97