[gn build] add llvm_install_cctools_symlinks arg

It behaves (mostly) like the LLVM_INSTALL_CCTOOLS_SYMLINKS option
in cmake.

The minor difference is that the llvm-objcopy symlinks bitcode_strip
and install_name_tool symlink to llvm-objcopy directly in the GN build,
while it's a bitcode_strip -> llvm-bitcode-strip -> objcopy chain
in the CMake build (and analogous for install_name_tool).

The implementation is very similar to the implementation of the
existing llvm_install_binutils_symlinks arg.

Differential Revision: https://reviews.llvm.org/D122312