Fix macOS fuzz: explicitly link LLVM libc++ to avoid weak-def symbol error

The fuzz targets were crashing on macOS 14 with:
  dyld: weak-def symbol not found '__ZnwmSt19__type_descriptor_t'

libFuzzer compiled with LLVM 18 uses typed allocation ABI symbols
not present in macOS 14's system libc++. Since DYLD_LIBRARY_PATH
cannot override SIP-protected /usr/lib/libc++.1.dylib at runtime,
we fix this at link time:
- -nostdlib++: suppress implicit system libc++ linking
- -L$LLVM/lib/c++ -lc++: explicitly link LLVM's libc++ (which has the symbol)
- -Wl,-rpath,$LLVM/lib/c++: embed rpath so dyld finds LLVM's libc++ at runtime

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>