Implement __truncxfhf2 in driver/runtime

http://b/26165616

As a WAR for this bug, define __truncxfhf2 in driver/runtime.  Note that
this implementation exhibits the double-rounding problem.  This WAR will
be removed once a proper implementation is added to compiler-rt.

Change-Id: Ic8ec8f86feff6da81c1c8e7c0b1d37112a20c609
1 file changed