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 #8604.
LGTM=rsc
R=golang-codereviews, bradfitz, rsc
CC=golang-codereviews
https://golang.org/cl/
137790043