[gn build] Pass -Wl,--export-dynamic for clang-repl

Fixes some clang-repl tests failing with
  error: Symbols not found: [ llvm_orc_registerJITLoaderGDBWrapper ]

Mirrors export_executable_symbols_for_plugins in CMake.  Other binaries
also use export_executable_symbols_for_plugins, but testing-wise it
seems like it's only necessary for clang-repl, so limit it to clang-repl
for now.