Correct -ftrapv to trap on errors, instead of calling the

__overflow_handler entrypoint that David Chisnall made up.
Calling __overflow_handler is not part of the contract of
-ftrapv provided by GCC, and should never have been checked
in in the first place.

According to:
http://permalink.gmane.org/gmane.comp.compilers.clang.devel/8699

David is using this for some of arbitrary precision integer stuff
or something, which is not an appropriate thing to implement on
this.

llvm-svn: 110490