interp: support GEP on fixed (MMIO) addresses

GetElementPtr would not work on values that weren't pointers. Because
fixed addresses (often used in memory-mapped I/O) are integers rather
than pointers in interp, it would return an error.

This resulted in the teensy40 target not compiling correctly since the
interp package rewrite. This commit should fix that.

interp/interpreter.go

@@ -550,7 +550,22 @@ func (r *runner) run(fn *function, params []value, parentMem *memoryView, indent
 			}
 			ptr, err := operands[0].asPointer(r)
 			if err != nil {
-				return nil, mem, r.errorAt(inst, err)
+				if err != errIntegerAsPointer {
+					return nil, mem, r.errorAt(inst, err)
+				}
+				// GEP on fixed pointer value (for example, memory-mapped I/O).
+				ptrValue := operands[0].Uint() + offset
+				switch operands[0].len(r) {
+				case 8:
+					locals[inst.localIndex] = literalValue{uint64(ptrValue)}
+				case 4:
+					locals[inst.localIndex] = literalValue{uint32(ptrValue)}
+				case 2:
+					locals[inst.localIndex] = literalValue{uint16(ptrValue)}
+				default:
+					panic("pointer operand is not of a known pointer size")
+				}
+				continue
 			}
 			ptr = ptr.addOffset(uint32(offset))
 			locals[inst.localIndex] = ptr