[tailscale1.17] golang.org/x/net/idna: add "omitidna" build tag to disable IDNA when unneeded

Updates golang#38311

Saves 640 KiB with the version in the Go 1.14 release branch:

dev:src $ ls -l idna.with*
-rwxr-xr-x 1 bradfitz bradfitz 1925839 Apr 14 08:31 idna.with
-rwxr-xr-x 1 bradfitz bradfitz 1269240 Apr 14 08:31 idna.without

dev:src $ gosize idna.with
1208154 (79.02%) runtime
148673 ( 9.72%) type
62866 ( 4.11%) reflect
22329 ( 1.46%) unicode
18606 ( 1.22%) golang.org/x/text/unicode/bidi
12578 ( 0.82%) sync
 8871 ( 0.58%) internal/reflectlite
 8123 ( 0.53%) golang.org/x/net/idna
 7446 ( 0.49%) syscall
 6337 ( 0.41%) golang.org/x/text/unicode/norm
 6312 ( 0.41%) internal/poll
 4771 ( 0.31%) os
 3415 ( 0.22%) internal/cpu
 1878 ( 0.12%) strconv
 1407 ( 0.09%) time
 1065 ( 0.07%) io
  881 ( 0.06%) fmt
  764 ( 0.05%) runtime/internal/sys
  650 ( 0.04%) sync/atomic
  497 ( 0.03%) runtime/debug
  475 ( 0.03%) golang.org/x/text/transform
  461 ( 0.03%) internal/oserror
  367 ( 0.02%) log
  349 ( 0.02%) unicode/utf8
  329 ( 0.02%) bytes
  307 ( 0.02%) errors
  277 ( 0.02%) golang.org/x/text/secure/bidirule
  185 ( 0.01%) internal/syscall/unix
  174 ( 0.01%) encoding/binary
  163 ( 0.01%) main
   77 ( 0.01%) internal/bytealg
   59 ( 0.00%) runtime/internal/atomic
   46 ( 0.00%) strings
   40 ( 0.00%) math
   14 ( 0.00%) internal/fmtsort
   11 ( 0.00%) sort
   11 ( 0.00%) internal/syscall/execenv

dev:src $ gosize idna.without
886959 (88.54%) runtime
101185 (10.10%) type
 8301 ( 0.83%) internal/reflectlite
 3391 ( 0.34%) internal/cpu
  496 ( 0.05%) runtime/debug
  476 ( 0.05%) runtime/internal/sys
  309 ( 0.03%) errors
  204 ( 0.02%) sync
  161 ( 0.02%) main
  145 ( 0.01%) golang.org/x/net/idna
   97 ( 0.01%) time
   77 ( 0.01%) internal/bytealg

Change-Id: Ibdc119694181ae3f6af0c607b39e865dc2325548

Author: josharian