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/memory.go

@@ -348,7 +348,7 @@ func (v literalValue) clone() value {
 }
 
 func (v literalValue) asPointer(r *runner) (pointerValue, error) {
-	return pointerValue{}, errLiteralToPointer
+	return pointerValue{}, errIntegerAsPointer
 }
 
 func (v literalValue) asRawValue(r *runner) rawValue {
@@ -949,7 +949,7 @@ func (v rawValue) clone() value {
 func (v rawValue) asPointer(r *runner) (pointerValue, error) {
 	if v.buf[0] <= 255 {
 		// Probably a null pointer or memory-mapped I/O.
-		return pointerValue{}, errExpectedPointer
+		return pointerValue{}, errIntegerAsPointer
 	}
 	return pointerValue{v.buf[0]}, nil
 }