Silence llbc's output by default to prevent rustc's linker output warning https://github.com/rust-lang/rust/pull/153968 recently made visible that the llvm-bitcode-linker (llbc) always emits messages to stdout even when linking successfully. Additionally, these messages use ANSI escape sequences for coloring, which don't get rendered correctly. This patch fixes both. - It silences llbc's output by default and adds a verbosity command line argument to it (`-v` for verbose and `-vv` for very verbose, which can be added by using rustc's `-Clink-arg=` argument). - It also removes the ANSI escape sequences and makes the output more linker-like (by removing timestamps, tracing-level, tracing-target). cc: @kjetilkjeka @rustbot label +L-linker_messages +O-NVPTX