runtime, reflect: avoid allocation in structType.Field common cases
Use assembler to make runtime.staticuint64s into a readonly array
so that the reflect package can safely create a slice without requiring
any allocation.
Fixes #2320
Fixes #68380
Change-Id: If2c97238eca782d0632db265c840581d4ecb9d18
Reviewed-on: https://go-review.googlesource.com/c/go/+/597855 Reviewed-by: Dmitri Shuralyov <dmitshur@google.com> Reviewed-by: Cherry Mui <cherryyz@google.com> Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Ian Lance Taylor <iant@golang.org>