i#1569 AArch64: Make dynamorio_syscall only read the 7th arg when used.

Fixes breakage in release-build AArch64 from d193665 exposing
dynamorio_syscall's deref of more args than were passed to it.

(Currently it seems that no Linux arm64 syscall has more than 6
arguments but several other Linux architectures do.)

Review-URL: https://codereview.appspot.com/308700043

core/arch/aarch64/aarch64_shared.asm

@@ -46,14 +46,20 @@ START_FILE
  */
         DECLARE_FUNC(dynamorio_syscall)
 GLOBAL_LABEL(dynamorio_syscall:)
+        cmp      w1, #7
         mov      x8,x0
         mov      x0,x2
         mov      x1,x3
         mov      x2,x4
         mov      x3,x5
         mov      x4,x6
         mov      x5,x7
+        /* We set up first 6 args unconditionally, but read 7th arg from stack
+         * only if there are at least 7 args.
+         */
+        b.cc     1f
         ldr      x6,[sp]
+1:
         svc      #0
         ret