kbuild: replace $(hdr-arch) with $(SRCARCH)

Since commit 5e53879008b9 ("sparc,sparc64: unify Makefile"), hdr-arch
and SRCARCH always match.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Douglas Anderson <dianders@chromium.org>