[dev.link] cmd/link: only do name expansion when needed

[dev.link] cmd/link: only do name expansion when needed

Most Go objects are compiled with known package path, so the
symbol name is already fully expanded. Nevertheless, currently
in the linker strings.Replace is called unconditionally, and most
of the time it doesn't do anything.

This CL records a per-object flag in the object file, and do the
name expansion only when the name is not expanded at compile time.

This gives small speedups for the linker. Linking cmd/compile:

name                    old time/op    new time/op    delta
Loadlib                   35.1ms ± 2%    32.8ms ± 4%   -6.43%  (p=0.008 n=5+5)
Symtab                    15.8ms ± 2%    14.0ms ± 8%  -11.45%  (p=0.008 n=5+5)
TotalTime                  399ms ± 1%     385ms ± 2%   -3.63%  (p=0.008 n=5+5)

Change-Id: I735084971a051cd9be4284ad294c284cd5b545f4
Reviewed-on: https://go-review.googlesource.com/c/go/+/234490
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jeremy Faller <jeremy@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>