aarch64: Try to free the GCS of makecontext

aarch64: Try to free the GCS of makecontext

Free GCS after a makecontext start func returns and at thread exit, so
assume makecontext cannot outlive the thread where it was created.

This is an attempt to bound the lifetime of the GCS allocated for
makecontext, but it is still possible to have significant GCS leaks,
new GCS aware APIs could solve that, but that would not allow using
GCS with existing code transparently.