jtw: fix jsoo_runtime parsing to handle newline-separated stubs
The core META file defines jsoo_runtime with newline-separated filenames,
but the parser only split on spaces. This left trailing newlines embedded
in file paths, causing js_of_ocaml to fail when compiling core.cma.js.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>