Use clock_gettime()'s CLOCK_REALTIME instead of gettimeofday().

The system_clock::now() function currently uses gettimeofday(). The
problem with gettimeofday() is that it is an obsolete XSI function,
hence unavailable on CloudABI. See:

http://pubs.opengroup.org/onlinepubs/9699919799/functions/gettimeofday.html

Change this code to use clock_gettime() with CLOCK_REALTIME instead,
which is more consistent, as clock_gettime() is already used for
steady_clock.

A previous version of this change actually attempted to change
system_clock::duration, but I reverted this part as it breaks the
existing ABI.

Differential Revision:	http://reviews.llvm.org/D8253
Approved by:	jroelofs

llvm-svn: 237390