internal/cpu: enable support for GODEBUGCPU in non-experimental builds

Enabling GODEBUGCPU without the need to set GOEXPERIMENT=debugcpu  enables
trybots and builders to run tests for GODEBUGCPU features in upcoming CLs
that will implement the new syntax and features for non-experimental
GODEBUGCPU support from proposal golang.org/issue/27218.

Updates #27218

Change-Id: Icc69e51e736711a86b02b46bd441ffc28423beba
Reviewed-on: https://go-review.googlesource.com/c/141817
Run-TryBot: Martin Möhrmann <moehrmann@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/internal/objabi/util.go

@@ -105,7 +105,6 @@ var (
 	Fieldtrack_enabled       int
 	Preemptibleloops_enabled int
 	Clobberdead_enabled      int
-	DebugCPU_enabled         int
 )
 
 // Toolchain experiments.
@@ -120,7 +119,6 @@ var exper = []struct {
 	{"framepointer", &framepointer_enabled},
 	{"preemptibleloops", &Preemptibleloops_enabled},
 	{"clobberdead", &Clobberdead_enabled},
-	{"debugcpu", &DebugCPU_enabled},
 }
 
 var defaultExpstring = Expstring()