[dev.link] cmd/link: pass reloc by value to Adddynrel2

[dev.link] cmd/link: pass reloc by value to Adddynrel2

Adddynrel2 is a function pointer. In dynrelocsym we pass &r to
it, which will cause r to escape. Pass it by value instead.

Linking cmd/compile,

name           old alloc/op   new alloc/op   delta
Dodata_GC        15.8MB ± 0%     5.9MB ± 0%  -62.55%  (p=0.008 n=5+5)

Change-Id: Ib86005d1026ebaca57777b27ead037e613585f44
Reviewed-on: https://go-review.googlesource.com/c/go/+/231045
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>