Add some minimal release notes for retpolines.

  ``__is_target_vendor``, ``__is_target_os``, and ``__is_target_environment``

  can be used to to examine the individual components of the target triple.

- Support for `retpolines <https://support.google.com/faqs/answer/7625886>`_

  was added to help mitigate "branch target injection" (variant #2) of the

  "Spectre" speculative side channels described by `Project Zero

  <https://googleprojectzero.blogspot.com/2018/01/reading-privileged-memory-with-side.html>`_

  and the `Spectre paper <https://spectreattack.com/spectre.pdf>`_.

Improvements to Clang's diagnostics

-----------------------------------

- New ``-nostdlib++`` flag to disable linking the C++ standard library. Similar

  to using ``clang`` instead of ``clang++`` but doesn't disable ``-lm``.

- Clang supports the ``-mretpoline`` flag to enable `retpolines

  <https://support.google.com/faqs/answer/7625886>`_. Code compiled with this

  flag will be hardened against variant #2 of the Spectre attack. Indirect

  branches from switches or gotos removed from the code, and indirect calls

  will be made through a "retpoline" thunk. The necessary thunks will

  automatically be inserted into the generated code. Clang also supports

  ``-mretpoline-external-thunk`` which works like ``-mretpoline`` but requires

  the user to provide their own thunk definitions. The external thunk names

  start with ``__x86_indirect_thunk_`` and end in a register name. For 64-bit

  platforms, only an ``r11`` thunk is used, but for 32-bit platforms ``eax``,

  ``ecx``, ``edx``, and ``edi`` thunks are used.

Attribute Changes in Clang

--------------------------