linux-user: Clean up target_syscall.h header guards

Some of them use guard symbol TARGET_SYSCALL_H, but we also have
CRIS_SYSCALL_H, MICROBLAZE_SYSCALLS_H, TILEGX_SYSCALLS_H and
__UC32_SYSCALL_H__.  They all upset scripts/clean-header-guards.pl.

Reuse of the same guard symbol TARGET_SYSCALL_H in multiple headers is
okay as long as they cannot be included together.  The script can't
tell, so it warns.

The script dislikes the other guard symbols, too.  They don't match
their file name (they should, to make guard collisions less likely),
and __UC32_SYSCALL_H__ is a reserved identifier.

Clean them all up: use guard symbol $target_TARGET_SYSCALL_H for
linux-user/$target/target_sycall.h.

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