Copy libsyntax from local Rust to stage0.

The local_stage0 script was not updated after commit 7dcbaed renamed
librustsyntax to libsyntax, so builds using local Rust fail due to
missing libsyntax.
auto merge of #5335 : larryv/rust/local_stage0-libsyntax, r=luqmana

The `local_stage0.sh` script was not updated after commit 7dcbaed renamed
librustsyntax to libsyntax.

Currently, `./configure --enable-local-rust --local-rust-root=FOO && make` will
fail due to the missing libsyntax; this change corrects this.