cmd/ld, cmd/5l, cmd/8l: support new 386/amd64 relocations

Backport of golang.org/cl/16529 to release-branch.go1.4.

I wanted to build go 1.4 packages for Xenial so I did this. It's not enough for
1.4.4 but it may as well be included if we ever find a reason to release that,
so I thought I'd send it in.

Change-Id: I907627586e6c7a0580ae447a349c0e3534d64e3d
Reviewed-on: https://go-review.googlesource.com/21598


Reviewed-by: Russ Cox <rsc@golang.org>