Previously, `serenity.sh rebuild-toolchain x86_64 clang` would simply
start building GCC instead and call it a day. With this change, if a
toolchain is passed as an argument, it is validated if it is either
"GNU" or "Clang" and exits with an error otherwise.
The `TOOLCHAIN` value defaults to the `SERENITY_TOOLCHAIN` environment
variable if not provided explicitly.