[libc] Setup TLS in x86_64 loader.

The new code added is still very x86_64 specific. AArch64 support will
be added very soon and refactoring of the loader code will be done as
part of the patches adding it.

Reviewed By: asteinhauser

Differential Revision: https://reviews.llvm.org/D82700

GitOrigin-RevId: 5d59385ba67ec20dc4a3e13b9a7088ace970df4d
7 files changed