hostcc/hostcxx: fix noccache variant setting

The current code accidentally sets up HOSTCC_NOCCACHE and HOSTCXX_NOCCACHE
only when the respective HOSTCC or HOSTCXX values are not set.  So if you
do something like:
	make HOSTCC=gcc menuconfig
The build fails because HOSTCC_NOCCACHE is not set anywhere.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>