SPEC2006: Allow building on FreeBSD
For FreeBSD, we also have to pass -DSPEC_CPU_MACOSX to avoid the use of
gcvt() in 400.perlbench as this function is not available on FreeBSD.
This define is also required to successfully build 462.libquantum and
483.xalancbmk since SPEC_CPU_BSD is not handled in those benchmarks.
Reviewed By: Meinersbur, fhahn
Differential Revision: https://reviews.llvm.org/D124393