Fix compiler-rt build on FreeBSD 10.1

We have to #define _WANT_RTENTRY to get struct rtentry, and add headers
that it depends on.

SIOCADDRT and SIOCDELRT also do not exist in FreeBSD 10.1 and were removed
in FreeBSD rev. 263203:

  Garbage collect long time obsoleted (or never used) stuff from routing
  API.

Differential Revision: http://reviews.llvm.org/D5703