pkg/coverage: adopt new coverage interface for kcov

 This change updates the kcov implementation to use the new Go coverage interface provided by the `internal/coverage` package. The previous implementation relied on `coverdata.Blocks`, which is part of the older coverage tooling.

The new implementation uses `runtime/coverage.WriteCounters` to get the raw coverage data and then decodes it using `decodecounter.NewDecoder`. This approach is aligned with the modern Go coverage infrastructure.

This change requires a patched Go compiler that allows importing internal packages. This is a temporary measure until the golang team provides a public API.

More details can be found here: golang/go#51430

FUTURE_COPYBARA_INTEGRATE_REVIEW=#12256 from avagin:kcov-v2-main 33081ed
PiperOrigin-RevId: 825669734

Author: avagin

pkg/coverage/BUILD

@@ -11,6 +11,7 @@ go_library(
     visibility = ["//:sandbox"],
     deps = [
         "//pkg/hostarch",
+        "//pkg/log",
         "//pkg/sync",
         "@io_bazel_rules_go//go/tools/coverdata",
     ],