cmd/genbootstrap: don't trim the Go 1.4 bootstrap archives so much
cmd/genbootstrap: don't trim the Go 1.4 bootstrap archives so much

Fixes golang/go#28483

Change-Id: I24b60735e786a80142454320f3ecbac65ad9c24f
Reviewed-on: https://go-review.googlesource.com/c/145977
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

golang/build,golang/build,golang/build,golang/build