[libc++abi] Add a silent terminate handler to libcxxabi.

The current std::terminate_handler pulls in some string code, some I/O
code, and more. Since it is automatically setup as the default, this
means that any trivial binary linking against libcxxabi will get this
extra bloat.

This patch allows disabling it as a build-time option, if you want to
avoid the extra bloat.

Patch by Tom Rybka!

Reviewers: EricWF

Subscribers: danalbert, llvm-commits, mgorny

Differential Revision: https://reviews.llvm.org/D28497

llvm-svn: 291946