[libcxxabi] Introduce a -fno-exceptions libc++abi libary variant

Description

[libcxxabi] Introduce a -fno-exceptions libc++abi libary variant

Currently there is only support for a -fno-exceptions libc++ build. This is
problematic for functions such as std::terminate() which are defined in
libc++abi and using any of those functions throws away most of the benefits
of using -fno-exceptions (code-size). This patch introduces a -fno-exceptions
libc++abi build to address this issue.

This new variant of libc++abi cannot be linked against any with-exceptions
code as some symbols necessary for handling exceptions are missing in this
library.

Differential revision: http://reviews.llvm.org/D20677

Reviewers: EricWF, mclow.lists, bcraig

Details