5l, 6l, 8l: correct ELFRESERVE diagnostic

5l, 6l, 8l: correct ELFRESERVE diagnostic

If the length of the interpreter string
pushes us over the ELFRESERVE limit, the
resulting error message will be comical.

I was doing some ELF tinkering with a
modified version of 8l when I hit this.

To be clear, the stock linkers wouldn't
hit this without adding about forty more
section headers.  We're safe for now. ;)

Also, remove a redundant call to cflush.

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/5268044