cmd/internal/obj/arm64: add SVC to unary destination list

To support "SVC $0x80", which is needed for darwin/arm64.

Change-Id: I3b3f80791a1db4c2b7318f81a115972cd2237f00
Signed-off-by: Shenghou Ma <minux@golang.org>
Reviewed-on: https://go-review.googlesource.com/8769Reviewed-by: David Crawshaw <crawshaw@golang.org>

src/cmd/internal/obj/arm64/obj7.go

@@ -811,6 +811,7 @@ var unaryDst = map[int]bool{
 	ADWORD: true,
 	ABL:    true,
 	AB:     true,
+	ASVC:   true,
 }
 
 var Linkarm64 = obj.LinkArch{