cmake: Support both toolchain directory layouts

Until https://github.com/zephyrproject-rtos/sdk-ng/pull/936

 is merged,
SDK 0.18 is incompatible with previous versions as the paths to
find the cmake bits is different.

Deal with that by checking for files in the wrong place as well as the
right one.

Signed-off-by: Keith Packard <keithp@keithp.com>