use named Registers accessors instead of direct esp/eax/etc.
push RDTSC register setting into Registers
don't invoke non-existent SYS_sigprocmask in tests
don't test non-existent SYS_fadvise64_64
avoid duplicate case values for F_GETLK{,64} and friends
remove static_assert in futex_wait by implementing Task::read_int