parisc: Allow CONFIG_64BIT with ARCH=parisc

The previous patch triggered a build failure for the debian kernel,
which has CONFIG_64BIT enabled, uses the CROSS_COMPILER environment
variable and uses ARCH=parisc to configure the kernel for 64-bit
support.

This patch weakens the previous patch while keeping the recommended way
to configure the kernel with:
    ARCH=parisc     -> build 32-bit kernel
    ARCH=parisc64   -> build 64-bit kernel
while adding the possibility for debian to configure a 64-bit kernel
even if ARCH=parisc is set (PA8X00 CPU has to be selected and
CONFIG_64BIT needs to be enabled).

The downside of this patch is, that we now have a small window open
again where people may get it wrong: if they enable CONFIG_64BIT and try
to compile with a 32-bit compiler.

Fixes: 3dcfb729 ("parisc: Make CONFIG_64BIT available for ARCH=parisc64 only")
Signed-off-by: Helge Deller <deller@gmx.de>
Cc: <stable@vger.kernel.org> # 5.15+