parisc: Fix compile failure when building 64-bit kernel natively

Commit 23243c1a ("arch: use cross_compiling to check whether it is
a cross build or not") broke 64-bit parisc builds on 32-bit parisc
systems.

Helge mentioned:
  - 64-bit parisc userspace is not supported yet [1]
  - hppa gcc does not support "-m64" flag [2]

That means, parisc developers working on a 32-bit parisc machine need
to use hppa64-linux-gnu-gcc (cross compiler) for building the 64-bit
parisc kernel.

After the offending commit, gcc is used in such a case because
both $(SRCARCH) and $(SUBARCH) are 'parisc', hence cross_compiling is
unset.

A correct way is to introduce ARCH=parisc64 because building the 64-bit
parisc kernel on a 32-bit parisc system is not exactly a native build,
but rather a semi-cross build.

[1]: https://lore.kernel.org/linux-parisc/5dfd81eb-c8ca-b7f5-e80e-8632767c022d@gmx.de/#t
[2]: https://lore.kernel.org/linux-parisc/89515325-fc21-31da-d238-6f7a9abbf9a0@gmx.de/

Fixes: 23243c1a

 ("arch: use cross_compiling to check whether it is a cross build or not")
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reported-by: Meelis Roos <mroos@linux.ee>
Tested-by: Meelis Roos <mroos@linux.ee>
Cc: <stable@vger.kernel.org> # v5.13+
Signed-off-by: Helge Deller <deller@gmx.de>