[libcxxabi] Don't process exceptions in cxa_handlers when they're disabled

When exceptions are disabled, avoid their processing altogether.
This avoids pulling in the depenency on demangler significantly
reducing binary size when statically linking against libc++abi
built without exception support.

Differential Revision: https://reviews.llvm.org/D64191

git-svn-id: https://llvm.org/svn/llvm-project/libcxxabi/trunk@365944 91177308-0d34-0410-b5e6-96231b3b80d8