cmd/go: include experiments in compiler tool ID

No test because testing this would require building a new toolchain
with a different experiment.

Fixes #26883

Change-Id: Iadd513d0920ef12463006dd2a61e94370dd13f68
Reviewed-on: https://go-review.googlesource.com/128735
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/go/internal/work/buildid.go

@@ -18,6 +18,7 @@ import (
 	"cmd/go/internal/load"
 	"cmd/go/internal/str"
 	"cmd/internal/buildid"
+	"cmd/internal/objabi"
 )
 
 // Build IDs
@@ -206,6 +207,11 @@ func (b *Builder) toolID(name string) string {
 		id = f[2]
 	}
 
+	// For the compiler, add any experiments.
+	if name == "compile" {
+		id += " " + objabi.Expstring()
+	}
+
 	b.id.Lock()
 	b.toolIDCache[name] = id
 	b.id.Unlock()