unix: add sendfile support on Solaris

Same was done in CL 7940 for the syscall package.

Fixes golang/go#10838

Change-Id: I98efa99f57f21bdcdbfb117af66f673e37767fec
Reviewed-on: https://go-review.googlesource.com/103335
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

unix/syscall_solaris_amd64.go

@@ -21,8 +21,3 @@ func (iov *Iovec) SetLen(length int) {
 func (cmsg *Cmsghdr) SetLen(length int) {
 	cmsg.Len = uint32(length)
 }
-
-func sendfile(outfd int, infd int, offset *int64, count int) (written int, err error) {
-	// TODO(aram): implement this, see issue 5847.
-	panic("unimplemented")
-}