lib9: build tokenize on Plan 9

The build was broken on Plan 9 after the
CL 2994, because of the use of getfields
in src/liblink/go.c.

This happened when building 8l, because
getfield was part of lib9 and tokenize
was part of the Plan 9 libc. However,
both getfields and tokenize depend on
utfrune, causing an incompatibility.

This change enables the build of tokenize
as part of lib9, so it doesn't use
tokenize from the Plan 9 libc anymore.

Change-Id: I2a76903b508bd92771c4754cd53dfc64df350892
Reviewed-on: https://go-review.googlesource.com/3121


Reviewed-by: Minux Ma <minux@golang.org>