cmake: toolchain: Don't warn of both ZEPHYR_*_VARIANT envvars defined

Someone doing bisects on Zephyr will have to have both older
ZEPHYR_GCC_VARIANT and newer ZEPHYR_TOOLCHAIN_VARIANT environment
variables defined. Then warning about ZEPHYR_GCC_VARIANT is
confusing. Issue it if only ZEPHYR_GCC_VARIANT is defined, and
ZEPHYR_TOOLCHAIN_VARIANT isn't.

Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>