cmd/internal/obj/arm64: make assembler almost concurrency-safe

CL 39922 made the arm assembler concurrency-safe.
This CL does the same, but for arm64.
The approach is similar: introduce ctxt7 to hold
function-local state and thread it through
the assembler as necessary.

One race remains after this CL, deep in aclass,
in the check that a Prog does not take the address
of a TLS variable.

That race is conceptually unrelated to this refactoring,
and will be addressed in a separate CL.

Passes toolstash-check -all.

Updates #15756

Change-Id: Icab1ef70008468f9a5b8bf728a77c4520bbcb67d
Reviewed-on: https://go-review.googlesource.com/40252
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

src/cmd/internal/obj/link.go

@@ -728,8 +728,6 @@ type Link struct {
 	Imports       []string
 	Plan9privates *LSym
 	Printp        *Prog
-	Blitrl        *Prog
-	Elitrl        *Prog
 	Instoffset    int64
 	Autosize      int32
 	Pc            int64