`assert(interruptID >=0)` is always true as `interruptID` is an unsigned
int.
This was causing compilation tests failures in GCC-8 with the following
error:
```sh
src/arch/riscv/interrupts.cc:47:32: error: comparison is always true due to limited range of data type [-Werror=type-limits]
assert(interruptID >= 0);
```
Change-Id: I356be78d7f75ea5d20d34768fb8ece0f746be2fc
47c4dad869