kbuild: update ARCH alias info for sparc

The supported alias for building sparc 32-bit is "sparc32",
not "sparc", so update the alias documentation for that.
Just using "sparc" produces a 64-bit config file.

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Acked-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>