[dev.simd] internal/buildcfg: enable SIMD GOEXPERIMENT for amd64

Since we are developing and testing this, the default is on.
This may still cause us a little headache when developing
on other-architecture laptops.

Change-Id: I9e9e5ea4ff2312c0c8385386b5012370f00dbfbd
Reviewed-on: https://go-review.googlesource.com/c/go/+/675735
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>

src/internal/buildcfg/exp.go

@@ -84,6 +84,7 @@ func ParseGOEXPERIMENT(goos, goarch, goexp string) (*ExperimentFlags, error) {
 		AliasTypeParams: true,
 		SwissMap:        true,
 		SyncHashTrieMap: true,
+		SIMD:            goarch == "amd64", // TODO remove this (default to false) when dev.simd is merged
 		Dwarf5:          dwarf5Supported,
 	}