um: clean up archheaders recipe

Now that '%asm-generic' is added to no-dot-config-targets,
'make asm-generic' does not include the kernel configuration.

You can simply do 'make asm-generic' in the recursed top Makefile
without bothering syncconfig.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Tested-by: Randy Dunlap <rdunlap@infradead.org>
Acked-by: Richard Weinberger <richard@nod.at>