Tweak to make clang/test/Driver/ps4-linker-win.c less sinsitive of DLL path.

  - On mingw-w64, libstdc++-6.dll is used for clang.exe. The DLL might not be in Windows' system directory.
  - With --enable-shared, DLLs might be in ${CMAKE_BINARY_DIR}/bin.

I understand this test confirms that appropriate name of executable can be found on %PATH%.
Therefore I added "Output\\" before each expression.

FIXME: The output directory %T is hardcoded like "Output\\ps4-ld.exe".
llvm-svn: 250403