[compiler-rt][CMake][arm64] Use a custom target for symlinking LSE sources

On Apple platforms the builtins may be built for both arm64 and arm64e.
With Makefile generators separate targets are built using Make sub-invocations.
This causes a race when creating the symlink which may sometimes fail.

Work around this by using a custom target that the builtin targets depend on.
This causes any sub-invocations to depend on the symlinks having been created before.

Mailing list thread: https://lists.llvm.org/pipermail/llvm-dev/2021-July/151822.html

Reviewed By: thakis, steven_wu

Differential Revision: https://reviews.llvm.org/D106305