This commit adds support for software vectoring in the PLIC interrupt.
The interrupt table is created by the compiler, which leads to very
compact code while retaining the flexibility that the interrupt API
provides.
This commit lets the compiler know about interrupts and allows
optimizations to be performed based on that: interrupts are eliminated
when they appear to be unused in a program. This is done with a new
pseudo-call (runtime/interrupt.New) that is treated specially by the
compiler.
This simplifies the code. The fields are blank anyway so there is no way
to access them anyway (volatile or not).
Also do some other related simplifications of the code that result from
this change.
This should make it more maintainable. Another big advantage that
generation time (including gofmt) is now 3 times faster. No real attempt
at refactoring has been made, that will need to be done at a later time.
