Previously, the builtin runtime export data was reparsed before every
Go source file, and the unsafe export data was reparsed for every
import of package unsafe.  Now, we parse both of them just once ahead
of time.

This does mean package unsafe's export data will be loaded even when
compiling packages that don't import it, but it's tiny anyway.

Change-Id: Ic6931bc58f6d62f664348bfa932f92d4ccacc3ef
Reviewed-on: https://go-review.googlesource.com/19626
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>