Allow llvm-# as a valid toolchain value
My apologies if this issue is not entirely accurate with the current state of things, this is based off a previous experience I had and the current documentation around valid toolchains.
LLVM 17 now supports full LLVM=1 LLVM_IAS=1 for ARCH=loongarch, support for which was added into tuxmake in tuxmake@a0456922. We would like to add support for ARCH=loongarch to ClangBuiltLinux's continuous integration.
However, as far as I can tell, there are no GNU binutils for loongarch64 in Debian yet, so allowing ARCH=loongarch for clang-17 and clang-18 in tuxmake would not work, as that is equivalent to CC=clang-17 CROSS_COMPILE=loongarch64-linux-gnu- (clang as the compiler and GNU binutils for everything else).
tuxmake supports llvm/llvm-# to specify LLVM=1 automatically, which would allow us to enable ARCH=loongarch for llvm-17 and llvm-nightly and nothing else. However, it appears that tuxsuite does not support llvm-# as a valid toolchain value. Can this be changed so that we can safely support ARCH=loongarch in tuxmake?