[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 <bfcdf3e6ca6cef45543bfbb57509c92aec9a39fb@cheney.net>
Reviewed-by: Aram Hăvărneanu <947f6c13826812af1df6a3fba06092a9b6c6b082@mgk.ro>
Reviewed-on: https://go-review.googlesource.com/14246
Run-TryBot: Chris Broadfoot <0c2851a4f8ab4aa5606e69c0757771dd13dbe686@golang.org>
Reviewed-by: Minux Ma <72d850dbd8858b791995463d8539b5a379577f28@golang.org>

willjunspecial/go,willjunspecial/go,willjunspecial/go,willjunspecial/go,willjunspecial/go,willjunspecial/go,willjunspecial/go,willjunspecial/go