build: let GNU ld find libLLVM's dependencies with a sysroot toolchain
LLVM's tools link against libLLVM, which depends on our zlib and zstd.
GNU ld looks for the dependencies of shared libraries in -rpath-link
directories and its default ones, not in -L directories, so with a
toolchain whose default search path is its own sysroot, linking fails
with undefined references to zlib/zstd functions.