arm64: vdso: fix and clean-up Makefile

- $(call if_changed,...) must have FORCE as a prerequisite

- vdso.lds is a generated file, so it should be prefixed with
  $(obj)/ instead of $(src)/.

- cmd_vdsosym is a one-liner rule, so the assignment with '='
  is simpler.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>