added gettimeofday syscall

RIOT's gettimeofday needs to be called from syscalls.c to assure that gcc actually links it.

If an RTC if available and enabled it will be used instead.
fixed typo
Adds test for pthread.

The test uses pthread to calculate the factorial of 12, with one
thread for each multiplication. pthread_mutex is used for synchronization.
Merge pull request #782 from OlegHahm/syscall_gettimeofday

sys: vtimer: added gettimeofday syscall
Merge pull request #756 from josephnoir/add_test_pthread_factorial

tests: Add test for pthread