cmd/internal/obj/arm: make assembler concurrency-safe

Move global state from obj.Link
to a new function-local state struct arm.ctxt5.

This ends up being cleaner than threading
all the state through as parameters; there's a lot of it.
While we're here, move newprog from a parameter to ctxt5.

We reserve the variable name c for ctxt5,
so a few local variables named c have been renamed.

Instead of lazily initializing deferreturn
and Sym_div and friends, initialize them up front.

Passes toolstash-check -all.

Updates #15756

Change-Id: Ifb4e4b9879e4e1f25e6168d8b7b2a25a3390dc11
Reviewed-on: https://go-review.googlesource.com/39922
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

@@ -726,17 +726,12 @@ type Link struct {
 	PosTable      src.PosTable
 	InlTree       InlTree // global inlining tree used by gc/inl.go
 	Imports       []string
-	Sym_div       *LSym
-	Sym_divu      *LSym
-	Sym_mod       *LSym
-	Sym_modu      *LSym
 	Plan9privates *LSym
 	Printp        *Prog
 	Blitrl        *Prog
 	Elitrl        *Prog
 	Instoffset    int64
 	Autosize      int32
-	Armsize       int32
 	Pc            int64
 	DiagFunc      func(string, ...interface{})
 	DebugInfo     func(fn *LSym, curfn interface{}) []*dwarf.Var // if non-nil, curfn is a *gc.Node