[release-branch.go1.5] net: add -lsendfile to cgo LDFLAGS for solaris

[release-branch.go1.5] net: add -lsendfile to cgo LDFLAGS for solaris

Fixes external linking of net/http tests (or anything that uses
sendfile).

Fixes #12390.

Change-Id: Iee08998cf66e7b0ce851db138a00ebae6dc2395e
Reviewed-on: https://go-review.googlesource.com/14072
Reviewed-by: Dave Cheney <dave@cheney.net>
Reviewed-by: Aram Hăvărneanu <aram@mgk.ro>
Reviewed-on: https://go-review.googlesource.com/14246
Run-TryBot: Chris Broadfoot <cbro@golang.org>
Reviewed-by: Minux Ma <minux@golang.org>