cmd/compile: randomize compilation order when race-enabled

cmd/compile: randomize compilation order when race-enabled

There's been one failure on the race builder so far,
before we started sorting functions by length.

The race detector can only detect actual races,
and ordering functions by length might reduce the odds
of catching some kinds of races. Give it more to chew on.

Updates #20144

Change-Id: I0206ac182cb98b70a729dea9703ecb0fef54d2d0
Reviewed-on: https://go-review.googlesource.com/41973
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>