internal/buildcfg: disable sizespecializedmalloc by default

We're finding some regressions in overall icache footprint at scale and
we should figure out how to deal with those before rolling this out to
everybody.

Change-Id: I98e792db31712bf64575d76dfeafedb48898f76a
Reviewed-on: https://go-review.googlesource.com/c/go/+/738780
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
Auto-Submit: Michael Knyszek <mknyszek@google.com>

src/internal/buildcfg/exp.go

@@ -81,12 +81,11 @@ func ParseGOEXPERIMENT(goos, goarch, goexp string) (*ExperimentFlags, error) {
 	dwarf5Supported := (goos != "darwin" && goos != "ios" && goos != "aix")
 
 	baseline := goexperiment.Flags{
-		RegabiWrappers:        regabiSupported,
-		RegabiArgs:            regabiSupported,
-		Dwarf5:                dwarf5Supported,
-		RandomizedHeapBase64:  true,
-		SizeSpecializedMalloc: true,
-		GreenTeaGC:            true,
+		RegabiWrappers:       regabiSupported,
+		RegabiArgs:           regabiSupported,
+		Dwarf5:               dwarf5Supported,
+		RandomizedHeapBase64: true,
+		GreenTeaGC:           true,
 	}
 	flags := &ExperimentFlags{
 		Flags:    baseline,