runtime: remove holes in FUNCDATA tables

Shrinks the text segment size by about 1.5% for the "go", "gofmt",
and "camlistored" commands on linux/amd64.

Before:
$ size go gofmt camlistored
   text	   data	    bss	    dec	    hex	filename
6506842	 136996	 105784	6749622	 66fdb6	go
2376046	  85232	  90984	2552262	 26f1c6	gofmt
17051050	 190256	 130320	17371626	10911ea	camlistored

After:
$ size go gofmt camlistored
   text	   data	    bss	    dec	    hex	filename
6403034	 136996	 105784	6645814	 656836	go
2331118	  85232	  90984	2507334	 264246	gofmt
16842586	 190256	 130320	17163162	105e39a	camlistored

Fixes issue 8604.

LGTM=rsc
R=golang-codereviews, bradfitz, rsc
CC=golang-codereviews
https://codereview.appspot.com/137790043

Committer: Ian Lance Taylor <iant@golang.org>