runtime/coverage: Support set mode

[r-hang]

Go's coverageredesign in a non unit set setting enforces that the program be instrumented in atomic coverage mode.
-

go/src/internal/coverage/cfile/apis.go

Line 39 in 6fbad4b

return fmt.Errorf("WriteCountersDir invoked for program built with -covermode=%s (please use -covermode=atomic)", cmode.String())

• https://cs.opensource.google/go/go/+/refs/tags/go1.24.5:src/runtime/coverage/coverage.go;l=64

Can we support set mode and provide performance estimates for for atomic mode? https://go.dev/blog/cover states that set mode can incur ~3% overhead.

One possible application of this feature is the deployment coverage instrumented binaries in production in order to detect and remove unused code. Atomic mode is prohibitively expensive to do on some production workloads.