Use LLVM_USE_SYMLINKS option in install_symlink

The change to potentially use symlinks on Windows was added in
https://reviews.llvm.org/D99170.

LLVM_USE_SYMLINKS was added more recently in
https://reviews.llvm.org/D135578 and allows specifying at configure time
whether or not symlinks should be created. The benefit of using this
option is it allows building the package on a symlink-capable Windows
machine with symlinks disabled so that the resulting package can be used
on a Windows machine that doesn't support symlinks.

Differential Revision: https://reviews.llvm.org/D145443