linux-user/nios2 linux-user/riscv: Clean up header guards

linux-user/nios2 linux-user/riscv: Clean up header guards

Reuse of the same guard symbol in multiple headers is okay as long as
they cannot be included together.  scripts/clean-header-guards.pl
can't tell, so it warns.

Since we can avoid guard symbol reuse easily, do so: use guard symbol
${target^^}_${fname^^} for linux-user/$target/$fname, just like we did
in commit a9c94277f0..3500385697.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20190315145123.28030-4-armbru@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>