[compiler-rt][Fuzzer] Use the new C++ ABI namespace CMake support

libc++ now supports customizing the ABI namespace directly from the
CMake build so we no longer need to rely on custom CFLAGS.

Differential Revision: https://reviews.llvm.org/D53939

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk/lib/fuzzer@345765 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed