runtime/secret: implement new secret package

runtime/secret: implement new secret package

Implement secret.Do.

- When secret.Do returns:
  - Clear stack that is used by the argument function.
  - Clear all the registers that might contain secrets.
- On stack growth in secret mode, clear the old stack.
- When objects are allocated in secret mode, mark them and then zero
  the marked objects immediately when they are freed.
- If the argument function panics, raise that panic as if it originated
  from secret.Do. This removes anything about the secret function
  from tracebacks.

For now, this is only implemented on linux for arm64 and amd64.

This is a rebased version of Keith Randalls initial implementation at
CL 600635. I have added arm64 support, signal handling, preemption
handling and dealt with vDSOs spilling into system stacks.

Fixes #21865

Change-Id: I6fbd5a233beeaceb160785e0c0199a5c94d8e520
Co-authored-by: Keith Randall <khr@golang.org>
Reviewed-on: https://go-review.googlesource.com/c/go/+/704615
Reviewed-by: Roland Shoemaker <roland@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Filippo Valsorda <filippo@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>