Simplify SYSTEM_CLOCK implementation.

2013-04-29  Janne Blomqvist  <jb@gcc.gnu.org>

        * intrinsics/system_clock (gf_gettime_mono): Use variable
        resolution for fractional seconds argument.
        (system_clock_4): Simplify, update for gf_gettime_mono change.
        (system_clock_8): Likewise.

From-SVN: r198391