[dev.link] cmd/link: reduce allocations in Preload

[dev.link] cmd/link: reduce allocations in Preload

Don't create loader.Syms until LoadFull (it will be gone soon
anyway.)

Preallocate loader.objSym array.

Don't create loader.values until preloading is done.

Linking cmd/compile:

name           old alloc/op   new alloc/op   delta
Loadlib_GC       36.2MB ± 0%    20.0MB ± 0%  -44.91%  (p=0.016 n=5+4)

Change-Id: I82eddcfa7fb8fc4e84e8174a47e59cf1183dd83b
Reviewed-on: https://go-review.googlesource.com/c/go/+/233341
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>
Reviewed-by: Jeremy Faller <jeremy@golang.org>