cmd/asm/internal/asm: fix a bug in ARM assembly encoding test

cmd/asm/internal/asm: fix a bug in ARM assembly encoding test

It is expected to test assembly code for ARMv5, ARMv6 and ARMv7
in cmd/asm/internal/asm/endtoend_test.go. But actually the loop
in "func TestARMEndToEnd(t *testing.T)" runs three times all
for ARMv5.

This patch fixes that bug and adds a new armv6.s which is only tested
with GOARM=6.

fixes #20465

Change-Id: I5dbf00809a47ace2c195335e2c9bdd768479aada
Reviewed-on: https://go-review.googlesource.com/43930
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>