cmd/go: preprocess PGO profiles

cmd/go: preprocess PGO profiles

Following the previous CL, now actually run the preprofile tool to create the
preprocessed output.

There is still no build cache integration, so the tool will run on every
build even if nothing has changed.

For #58102.

Cq-Include-Trybots: luci.golang.try:gotip-linux-amd64-longtest
Change-Id: I0414377a956889f457e50898737fcaa8a698658d
Reviewed-on: https://go-review.googlesource.com/c/go/+/569424
Auto-Submit: Michael Pratt <mpratt@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>