runtime/secret: reorganize tests to fix -buildmode=shared

The testing assembly methods had a linkname that was implicitly
satisfied during the regular build but not there during the shared
build. Fix by moving the testing routine into the package itself.

For good measure, section off the assembly files from the non-experiment
build. Should prevent further build failures as we work on this.

Cq-Include-Trybots: luci.golang.try:gotip-linux-amd64-longtest,gotip-linux-arm64-longtest
Change-Id: I2b45668e44641ae7880ff14f6402d982c7eaedd7
Reviewed-on: https://go-review.googlesource.com/c/go/+/724001
Reviewed-by: David Chase <drchase@google.com>
Auto-Submit: Filippo Valsorda <filippo@golang.org>
Reviewed-by: Filippo Valsorda <filippo@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>

Author: DanielMorsing

src/runtime/secret/asm_amd64.s

@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+//go:build goexperiment.runtimesecret
+
 // Note: this assembly file is used for testing only.
 // We need to access registers directly to properly test
 // that secrets are erased and go test doesn't like to conditionally
@@ -208,6 +210,6 @@ noavx512:
 	// registers contain secrets.
 	// It also tests the path from G stack to M stack
 	// to scheduler and back.
-	CALL	·delay(SB)
+	CALL	runtime∕secret·delay(SB)
 
 	RET

src/runtime/secret/asm_arm64.s

@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+//go:build goexperiment.runtimesecret
+
 // Note: this assembly file is used for testing only.
 // We need to access registers directly to properly test
 // that secrets are erased and go test doesn't like to conditionally

src/runtime/secret/stubs.go

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-//go:build arm64 || amd64
+//go:build goexperiment.runtimesecret && (arm64 || amd64)
 
 // testing stubs, these are implemented in assembly in
 // asm_$GOARCH.s
@@ -30,3 +30,14 @@ func spillRegisters(p unsafe.Pointer) uintptr
 //
 //go:noescape
 func useSecret(secret []byte)
+
+// callback from assembly
+func delay() {
+	sleep(1_000_000)
+}
+
+// linknamed to avoid package importing time
+// for just testing code
+//
+//go:linkname sleep time.Sleep
+func sleep(int64)

src/runtime/secret/testdata/crash.go

@@ -18,13 +18,6 @@ import (
 	"weak"
 )
 
-// callback from assembly
-//
-//go:linkname delay main.delay
-func delay() {
-	time.Sleep(1 * time.Millisecond)
-}
-
 // Same secret as in ../../crash_test.go
 var secretStore = [8]byte{
 	0x00,