internal/goexperiment: add a sizespecializedmalloc goexperiment setting

Change-Id: I6a6a696497f2a0b0d403bbb11d7502f62edec78b
Reviewed-on: https://go-review.googlesource.com/c/go/+/696535
Auto-Submit: Michael Matloob <[email protected]>
Reviewed-by: Michael Knyszek <[email protected]>
Reviewed-by: Michael Matloob <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>

Author: matloob

src/internal/goexperiment/exp_sizespecializedmalloc_off.go

@@ -115,4 +115,7 @@ type Flags struct {
 	// RandomizedHeapBase enables heap base address randomization on 64-bit
 	// platforms.
 	RandomizedHeapBase64 bool
+
+	// SizeSpecializedMalloc enables malloc implementations that are specialized per size class.
+	SizeSpecializedMalloc bool
 }