cmd/compile: go1.27.0 unified export data loses method order, breaking objectpath, crashing staticcheck [1.27 backport]

[gopherbot]

@adonovan requested issue #81188 to be considered for backport to the next 1.27 minor release.

@gopherbot Please create a backport issue for Go 1.27: the 1.27.0 compiler's export data causes applications to crash.

[gopherbot]

Change https://go.dev/cl/826644 mentions this issue: [internal-branch.go1.27-vendor] internal/gcimporter: support V5 unified encoding (method indices)