Bug 1480558 - part 3 - unset AS_DASH_C_FLAG for aarch64 windows; r=mshal

The assembler for this platform doesn't need the special handling
AS_DASH_C_FLAG provides.

old-configure.in

@@ -864,6 +864,10 @@ case "$target" in
         if test "$AS_BIN"; then
             AS="$(basename "$AS_BIN")"
         fi
+        # armasm64 doesn't understand -c.
+        if test "$CPU_ARCH" = "aarch64"; then
+            AS_DASH_C_FLAG=
+        fi
         case "$LINKER" in
         *lld*)
             AR='llvm-lib'