kbuild: replace $(LDFLAGS) $(ldflags-y) with $(ld_flags)

$(LDFLAGS) $(ldflags-y) is equivalent to $(ld_flags).

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>