parisc: Make CONFIG_64BIT available for ARCH=parisc64 only

With this patch the ARCH= parameter decides if the
CONFIG_64BIT option will be set or not. This means, the
ARCH= parameter will give:

	ARCH=parisc	-> 32-bit kernel
	ARCH=parisc64	-> 64-bit kernel

This simplifies the usage of the other config options like
randconfig, allmodconfig and allyesconfig a lot and produces
the output which is expected for parisc64 (64-bit) vs. parisc (32-bit).

Suggested-by: Masahiro Yamada <masahiroy@kernel.org>
Signed-off-by: Helge Deller <deller@gmx.de>
Tested-by: Randy Dunlap <rdunlap@infradead.org>
Reviewed-by: Randy Dunlap <rdunlap@infradead.org>
Cc: <stable@vger.kernel.org> # 5.15+