kconfig: use $K64BIT to set 64BIT with all*config targets

The variable K64BIT can now be used to select the
value of CONFIG_64BIT.

This is for example useful for powerpc to generate
allmodconfig for both bit sizes - like this:
make ARCH=powerpc K64BIT=y
make ARCH=powerpc K64BIT=n

To use this the Kconfig file must use "64BIT" as the
config value to select between 32 and 64 bit.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Cc: Roman Zippel <zippel@linux-m68k.org>