trace: support BSD version of sed in gen.bash

The GNU version of sed (common on Linux) will modify files in place when
it sees the -i flag with no argument at all, as in [sed -i -e '...'].
The BSD version of sed (common on macOS) will modify files in place when
it's given an explicit zero-length argument, as in [sed -i '' -e '...'].

But they agree on at least one way of specifying a non-zero-length
backup suffix, [sed -i '.tmp' -e '...']. Use that and then delete the
backups.

In addition, change the expressions we pass to sed to make them a bit
more explicit and to require less escaping.

Fixes golang/go#68403

Change-Id: Icfc3863f4abe26bc77d9c0bc3b778067db6387de
Reviewed-on: https://go-review.googlesource.com/c/exp/+/598016
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Carlos Amedee <carlos@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Rhys Hiltner <rhys.hiltner@gmail.com>
Reviewed-by: Mauri de Souza Meneguzzo <mauri870@gmail.com>