target-*: Clean up cpu.h header guards

Most of them use guard symbols like CPU_$target_H, but we also have
__MIPS_CPU_H__ and __TRICORE_CPU_H__.  They all upset
scripts/clean-header-guards.pl.

The script dislikes CPU_$target_H because they don't match their file
name (they should, to make guard collisions less likely).  The others
are reserved identifiers.

Clean them all up: use guard symbol $target_CPU_H for
target-$target/cpu.h.

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