[cmake] Don't include symlinks to tools in Build-all when `LLVM_BUILD_TOOLS` is off

When building LLVM with LLVM_BUILD_TOOLS as OFF, numerous tools such as llvm-ar or llvm-objcopy end up still being built. The reason for this is that the symlink targets are unconditionally included in a Build-all build, causing the tool they're symlinking to be built after all.

This patch changes that behaviour to be more intuitive by only including the symlink in a Build-all build if the target they're linking to is also included.

Differential Revision: https://reviews.llvm.org/D132883