Enabling loongarch for clang-nightly

LLVM 17 (release/17.x, packages for which are not available on apt.llvm.org yet, I will open an issue for that when the time comes) and LLVM 18 (current main/nightly) now support full LLVM=1 for ARCH=loongarch. I would like to add support for this architecture/toolchain combination to TuxMake so that we can start testing this in our continuous integration.

Unfortunately, Debian does not appear to have packaged GNU binutils for LoongArch yet (https://packages.debian.org/search?keywords=binutils&searchon=names&suite=all&section=all), so as far as I can tell, enabling loongarch for clang-nightly would not work (as that uses CC=clang CROSS_COMPILE=loongarch64-linux-gnu-). Only llvm-nightly would currently work but the clang and llvm values are tightly coupled, so enabling just llvm for an architecture does not appear to be possible. The only other situation that is somewhat similar to this is Hexagon but Hexagon has no support in the GNU toolchain altogether, whereas LoongArch does (it is just not packaged in Debian yet), so it seems wrong to unconditionally add LLVM=1 in tuxmake/arch/loongarch.ini, especially since I tested the initial support for LoongArch with GCC from https://kernel.org/pub/tools/crosstool.

Some guidance about how to go about this would be greatly appreciated! cc @nickdesaulniers