cmd/compile, x/build/cmd/relui: update compiler's PGO profile as part of rc1 relui flow

[cherrymui]

CL https://golang.org/cl/451292 adds a default profile of the compiler, so we can build the compiler with PGO. As the compiler's source code evolves, we may want to periodically update the profile.

cc @prattmic @aclements

[aclements]

We discussed this in the release team meeting today. The plan is to add this as a relui step, to be performed as part of creating rc1. We can also run the step as a one-off if necessary (e.g., after any major compiler refactoring).

Ideally we can shrink the profile a little. Currently it's ~350K, which is just big enough to be a little annoying to update regularly in git.

cc @golang/release

[heschi]

We should discuss whether to try to get it done for this cycle. On the one hand it's not very necessary, but on the other it might be best to strike while the iron is hot.

[cherrymui]

Ideally we can shrink the profile a little

When I did it I put GOROOT in /tmp so the file paths are short. We could probably also trim cold nodes from the profile to reduce size.