Add get_absolute_time() syscall + implement std::chrono::steady_clock::now() using it.

Add get_absolute_time() syscall + implement std::chrono::steady_clock::now() using it.

This addresses T64.  Needs revising and more work to make it available on
architectures other than x86_64.

This may not quite satisfy the requirements mandated for
std::chrono::steady_clock, but is good enough to make steady_clock workable
for code that needs to work with millisecond precision durations.

Signed-off-by: Simon Gerber <simon.gerber@inf.ethz.ch>