When the WASM_OPT environment variable is set, run the wasm-opt tool to optimize LLVM's output. Note that using wasm-opt currently means invalidating all debug info, however we can just tell users this when we tell them about WASM_OPT. This fixes PR43796.
Also, add an "llvm-lto" directory to the sysroot library search paths, so that sysroots can optionally provide LTO-enabled system libraries.