Include stddef.h before including cxxabi.h

On FreeBSD 10.0, size_t needs to be defined before including cxxabi.h.
Currenty HAVE_CXXABI_H is not defined on FreeBSD because of that reason.
This patch teaches cmake and configure how to include it.

http://reviews.llvm.org/D5940
Remove #ifdef __FreeBSD

r220665 makes configure or CMake to set HAVE_CXXABI_H on FreeBSD 10.0.
We don't need this ugly ifdef condition anymore.
Revert "Include stddef.h before including cxxabi.h" to un-break buildbot

This reverts commit r220665.