We already explicitly construct the "unsafe.Pointer" type in typeinit
because we need it for Types[TUNSAFEPTR]. No point in also having it
in builtin/unsafe.go if it just means (*importer).importtype needs to
fix it.

Change-Id: Ife8a5a73cbbe2bfcabe8b25ee4f7e0f5fd0570b4
Reviewed-on: https://go-review.googlesource.com/29082
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>