cmd/internal/obj/arm: don't split instructions on NaCl

We insert guard instructions after each "dangerous" instruction
to make NaCl's validator happy. This happens before asmout. If
in asmout an instruction is split to two dangerous instructions,
but only one guard instruction is inserted, the validation fails.
Therefore don't split instructions on NaCl.

Fixes #20595.

Change-Id: Ie34f209bc7d907d6d16ecef6721f88420981ac01
Reviewed-on: https://go-review.googlesource.com/45021
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/internal/obj/arm/asm5.go

@@ -1262,12 +1262,16 @@ func (c *ctxt5) aclass(a *obj.Addr) int {
 			if uint32(c.instoffset) <= 0xffff && objabi.GOARM == 7 {
 				return C_SCON
 			}
+			if c.ctxt.Headtype != objabi.Hnacl {
+				// Don't split instructions on NaCl. The validator is not
+				// happy with it. See Issue 20595.
 				if x, y := immrot2a(uint32(c.instoffset)); x != 0 && y != 0 {
 					return C_RCON2A
 				}
 				if y, x := immrot2s(uint32(c.instoffset)); x != 0 && y != 0 {
 					return C_RCON2S
 				}
+			}
 			return C_LCON
 
 		case obj.NAME_EXTERN,