[compiler-rt] Build custom libc++abi without exceptions.
Since neither compiler-rt nor the libc++ we build use exceptions, we
don't need libc++abi to have them either.
This resolves an issue where libFuzzer's private libc++ contains
implementations for __cxa_throw and friends, causing fuzz targets built
with their own C++ library to segfault during exception unwinding.
Reviewers: phosek, EricWF, kcc
Reviewed By: phosek
Subscribers: kcc, dberris, mgorny, christof, llvm-commits, metzman
Differential Revision: https://reviews.llvm.org/D61053