[release-branch.go1.25] runtime: initialise debug settings much earlier in startup...

[release-branch.go1.25] runtime: initialise debug settings much earlier in startup process

This is necessary specifically to set the value of `debug.decoratemappings`
sufficiently early in the startup sequence that all memory ranges allocated
can be named appropriately using the new Linux-specific naming API
introduced in #71546.

Example output (on ARM64):
https://gist.github.com/9muir/3667654b9c3f52e8be92756219371672

For: #75324
Fixes #75669

Change-Id: Ic0b16233f54a45adef1660c4d0df59af2f5af86a
Reviewed-on: https://go-review.googlesource.com/c/go/+/703476
Auto-Submit: Michael Knyszek <mknyszek@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
(cherry picked from commit 300d9d2714164e455abc7990d52de9de6b084df1)
Reviewed-on: https://go-review.googlesource.com/c/go/+/708359
Reviewed-by: Cherry Mui <cherryyz@google.com>