Normalize position of header guard

Normalize position of header guard

This is the common header guard idiom:

    /*
     * File comment
     */

    #ifndef GUARD_SYMBOL_H
    #define GUARD_SYMBOL_H

    ... actual contents ...

    #endif

A few of our headers have some #include before the guard.
target/tilegx/spr_def_64.h has #ifndef __DOXYGEN__ outside the guard.
A few more have the #define elsewhere.

Change them to match the common idiom.  For spr_def_64.h, that means
dropping #ifndef __DOXYGEN__.  While there, rename guard symbols to
make scripts/clean-header-guards.pl happy.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20190604181618.19980-2-armbru@redhat.com>
[Rebased with conflicts resolved automatically]