Allow using Hexagon with kernel.org LLVM

Attempting to use Hexagon with korg-clang or korg-llvm results in:

$ tuxmake -a hexagon -k defconfig -r podman -t korg-clang-18 LLVM=1 default
E: Unsupported architecture/toolchain combination: hexagon/korg-clang-18

which should be allowed, as the kernel.org toolchains support Hexagon just like the Debian ones do.