[avr] Removed usage of deprecated MCUSR

Since avr-libc 1.8.0 MCUSR is marked as poison [1] as it was replaced by the
correct name MCUCSR [2].
Thus code still using the old MCUSR name does not compile anymore.

This commit replaces usages of former MCUSR by its new name MCUCSR and
modifies the alias fallback accordingly.

[1] http://svn.savannah.nongnu.org/viewvc/trunk/avr-libc/include/avr/iom128.h?root=avr-libc&r1=2225&r2=2226
[2] http://svn.savannah.nongnu.org/viewvc/trunk/avr-libc/include/avr/iom128.h?root=avr-libc&r1=2&r2=61&pathrev=2226