parisc: enhance automatic CONFIG_CROSS_COMPILE detection

The current Makefile will only choose the hppa64 cross compiler when
running natively on hppa in a 32bit userspace.
This patch additionally chooses the correct 32/64 bit hppa compiler even
when doing real cross compiling to hppa/hppa64 from another architecture.

Signed-off-by: Helge Deller <deller@gmx.de>