cmd/compile: seed rand with time when race enabled

cmd/compile: seed rand with time when race enabled

When the race detector is enabled,
the compiler randomizes the order in which functions are compiled,
in an attempt to shake out bugs.
But we never re-seed the rand source, so every execution is identical.
Fix that to get more coverage.

Change-Id: If5cdde03ef4f1bab5f45e07f03fb6614945481d7
Reviewed-on: https://go-review.googlesource.com/43572
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>