Make rlimit-related syscalls use Arch::rlimit(64).
Add rlimit test.
Add ALLOCATE_GUARD and VERIFY_GUARD to help catch erroneous writes.

When we move to 64-bit builds by default, certain kinds of rr bugs could
result in too many bytes being written to user-space memory by a system
call (due to a too-large copy being performed from scratch memory to
the system call parameter pointer). To catch these, we should improve
our tests so that system call parameter buffers are allocated with
canary values on either side.
Use ALLOCATE/VERIFY_GUARD in rlimit.c
Make 'times' test use ALLOCATE/VERIFY_GUARD.
Add test for getrusage.
Add test for gettimeofday.
Make gettimeofday timezone parameter use Arch::timezone.

This doesn't really matter since it's obsolete and the
definition is the same across architectures anyway.
Make statfs syscalls use Arch::statfs(64).
Use ALLOCATE/VERIFY_GUARD in statfs test.
Make setitimer use Arch::itimerval.
Add test for setitimer/getitimer and implement getitimer.
Reformat