zephyr/defines: Move KERNEL and __ZEPHYR__ to toolchains Moves the KERNEL and __ZEPHYR__ defines out of the zephyr_cc_library and into the toolchain definition instead so that it's globally available. Change-Id: Id5ebe2d3a59f02feffdcadb1165a0a318f707886 Reviewed-on: https://pigweed-review.googlesource.com/c/zephyr/zephyr-bazel/+/435155