[dev.link] cmd/internal/obj: traverse files in deterministic order

[dev.link] cmd/internal/obj: traverse files in deterministic order

CL 245485 introduced a map for used files in a function. When
numbering symbols, make sure we traverse the files in
deterministic order.

Should fix longtest builders.

Change-Id: I1006bc5425116ab40e33a61e8f5acd1bdb4abad9
Reviewed-on: https://go-review.googlesource.com/c/go/+/247997
Run-TryBot: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>