[dev.link] cmd/link: use shared reloc slice in writelines method

[dev.link] cmd/link: use shared reloc slice in writelines method

Move to a shared/reused slice of loader.Reloc's in a couple of places
in the linker's DWARF writelines method, as opposed to allocating a
new slice each time. Small performance improvement.

Change-Id: I06c64cf7ca00162d24d9421f1d6405c8808ebb83
Reviewed-on: https://go-review.googlesource.com/c/go/+/222758
Run-TryBot: Than McIntosh <thanm@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>