kernel: Explicitly include toolchain.h to check for endianness definitions.

This commit adds an explicit inclusion of toolchain.h from kernel.h.

The endianness preprocessor definitions (__BYTE_ORDER__,
__ORDER_LITTLE_ENDIAN__, __ORDER_BIG_ENDIAN__) are used by kernel.h;
these being not defined can easily go unnoticed and cause unexpected
behaviours, as detailed in PR #18922.

toolchain.h ensures that these preprocessor definitions are defined
and *must* be included in a file that uses these definitions.

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>