[dev.simd] internal/buildcfg: turn GOEXPERIMENT=simd back on

Turn the experiment back on by default on the dev.simd branch, for
the ease of experimenting and development.

Change-Id: I5f7e945d55d9d2163e2730b15aea471270599550
Reviewed-on: https://go-review.googlesource.com/c/go/+/723942
Reviewed-by: Junyang Shao <shaojunyang@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

src/internal/buildcfg/exp.go

@@ -83,6 +83,7 @@ func ParseGOEXPERIMENT(goos, goarch, goexp string) (*ExperimentFlags, error) {
 	baseline := goexperiment.Flags{
 		RegabiWrappers:        regabiSupported,
 		RegabiArgs:            regabiSupported,
+		SIMD:                  goarch == "amd64", // TODO: remove this (default to false) when dev.simd is merged
 		Dwarf5:                dwarf5Supported,
 		RandomizedHeapBase64:  true,
 		SizeSpecializedMalloc: true,