tcg: Clean up tcg-target.h header guards

These use guard symbols like TCG_TARGET_$target.
scripts/clean-header-guards.pl doesn't like them because they don't
match their file name (they should, to make guard collisions less
likely).

Clean them up: use guard symbol $target_TCG_TARGET_H for
tcg/$target/tcg-target.h.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Richard Henderson <rth@twiddle.net>
