[dev.link] cmd/internal/goobj2: change StringRef encoding for better locality

Previously, StringRef is encoded as an offset pointing to
{ len, [len]byte }. This CL changes it to { len, offset }, where
offset points the bytes.

With the new format, reading a string header is just reading two
adjacent uint32s, without accessing the string table. This should
improve locality of object file reading.

Change-Id: Iec30708f9d9adb2f0242db6c4767c0f8e730f4df
Reviewed-on: https://go-review.googlesource.com/c/go/+/226797


Run-TryBot: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Jeremy Faller <jeremy@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>