[dev.simd] internal/goexperiment: add SIMD goexperiment

We'll use it to guard the simd package, and the compiler's
handling of SIMD types and intrinsics.

Change-Id: I0356368eea0a98a5016baaaf7acb7da8b6305429
Reviewed-on: https://go-review.googlesource.com/c/go/+/675536
Reviewed-by: David Chase <drchase@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

src/internal/goexperiment/exp_simd_off.go

@@ -0,0 +1,8 @@
+// Code generated by mkconsts.go. DO NOT EDIT.
+
+//go:build !goexperiment.simd
+
+package goexperiment
+
+const SIMD = false
+const SIMDInt = 0

src/internal/goexperiment/exp_simd_on.go

@@ -0,0 +1,8 @@
+// Code generated by mkconsts.go. DO NOT EDIT.
+
+//go:build goexperiment.simd
+
+package goexperiment
+
+const SIMD = true
+const SIMDInt = 1

src/internal/goexperiment/flags.go

@@ -129,4 +129,8 @@ type Flags struct {
 
 	// GreenTeaGC enables the Green Tea GC implementation.
 	GreenTeaGC bool
+
+	// SIMD enables the simd package and the compiler's handling
+	// of SIMD intrinsics.
+	SIMD bool
 }