[dev.link] cmd/internal/goobj2, cmd/link: change data type for local object/index...

[dev.link] cmd/internal/goobj2, cmd/link: change data type for local object/index representation

Use uint32 consistently for local index (this is what the object
file uses).

Use a index, instead of a pointer, to refer to the object file.
This reduces memory usage and GC work.

This reduces some allocations. Linking cmd/compile,

name           old alloc/op   new alloc/op   delta
Loadlib_GC       19.9MB ± 0%    16.9MB ± 0%  -15.33%  (p=0.008 n=5+5)

name           old live-B     new live-B     delta
Loadlib_GC        12.6M ± 0%     11.3M ± 0%   -9.97%  (p=0.008 n=5+5)

Change-Id: I20ce60bbb6d31abd2e9e932bdf959e2ae840ab98
Reviewed-on: https://go-review.googlesource.com/c/go/+/233779
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>