Static initialization order fiasco

[danluu]

Register.h

    // TODO: Need to avoid "static initialization order fiasco" for register definitions.
    // See http://www.parashift.com/c++-faq/static-init-order.html.
    // Plan is to use constexpr when VS2013 becomes available to Bing build.

Constexpr is available for us.

[danluu]

Below the comment, each extern line runs a constructor to define a register (like al). Constructors of random top-level things aren't always run first.