Improve performance of gettimeofday

Applied fixes to gettimeofday based on knowledge in this thread:
    http://www.freelists.org/post/luajit/simple-change-causes-trace-aborts,1

 * Lifted the timeval allocation to a local.
 * Put parentheses around the return value calculation
 * Replaced occurrences of ffi.C with local 'C'.

In particular, the lack of parentheses around the return value calculation could cause traces to abort, which hurts performance beyond just that function.