Use exclusively clock_gettime to implement gettimeofday.

Use exclusively clock_gettime to implement gettimeofday.

In 2.31 we changed most ports to implement gettimeofday using
clock_gettime(CLOCK_REALTIME[_COARSE]), but we kept around a handful
of Linux-specific implementations that went directly to the vDSO.

Actual performance testing (on x86-64) indicates that there is no
measurable difference between invoking the vDSO directly and calling
clock_gettime (which then calls into the vDSO), so remove all of these
for simplicity’s sake.