bionic: Optimizations for gettimeofday and clock_gettime.

Use the kernel user helper feature to calculate the results
of gettimeofday and clock_gettime without actually calling
into kernel space. If the user helper patches have not been
applied to the kernel, the regular system calls are used as
a fallback.

Change-Id: I3aebc6ac19ab4743725648a1a279819e624cc5c4