[dev.link] cmd/link: remove elderly ld bug workaround on arm64

[dev.link] cmd/link: remove elderly ld bug workaround on arm64

The arm64 archreloc method contains a workaround for a 2015-era
binutils/linker bug, https://sourceware.org/bugzilla/show_bug.cgi?id=18270.
This bug has been fixed for some time now, so remove the workaround
for it (the workaround includes some code that mutates a relocation
type, which is something we want to void doing in the new linker).

Change-Id: I9b7584e4daad240bbb85de673d704731705c8148
Reviewed-on: https://go-review.googlesource.com/c/go/+/232200
Run-TryBot: Than McIntosh <thanm@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>