cmd/asm: fix SIMD register name on arm64

Current V-register range is V32~V63 on arm64. This patch changes it to
V0~V31.

fix #15465.

Change-Id: I90dab42dea46825ec5d7a8321ec4f6550735feb8
Reviewed-on: https://go-review.googlesource.com/22520Reviewed-by: Aram Hăvărneanu <aram@mgk.ro>
Run-TryBot: Aram Hăvărneanu <aram@mgk.ro>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/internal/obj/arm64/list7.go

@@ -71,7 +71,7 @@ func Rconv(r int) string {
 	case REG_F0 <= r && r <= REG_F31:
 		return fmt.Sprintf("F%d", r-REG_F0)
 	case REG_V0 <= r && r <= REG_V31:
-		return fmt.Sprintf("V%d", r-REG_F0)
+		return fmt.Sprintf("V%d", r-REG_V0)
 	case COND_EQ <= r && r <= COND_NV:
 		return strcond[r-COND_EQ]
 	case r == REGSP: