sparc: Use 64BIT config entry

Use 64BIT config entry to distinguish between 32 and 64bit builds
instead of relying on the ARCH setting.  Using sparc64 as ARCH still
forces 64BIT on.

Inspired by the x86 and s390 configs.

[ Integrated CONFIG_64BIT help text suggestions from Sam -DaveM ]

Signed-off-by: Bastian Blank <waldi@debian.org>
Tested-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: David S. Miller <davem@davemloft.net>