Document __FreeBSD_version values:

• 1101509: backport of the LinuxKPI from FreeBSD-head
• 1101510: cmpxchg() macro is now fully functional in the LinuxKPI
• 1101511: concluding the recent LinuxKPI related updates
• 1101512: merging retpoline support from the upstream llvm, clang and lld 5.0 branches
• 1101513: merging clang, llvm, lld, lldb, compiler-rt and libc++ 6.0.0 release, and several follow-up fixes