[FreeBSD] Enable procmaps on FreeBSD.

This is not the final patch for procmaps, but it's enough for building
sanitizers and tests on FreeBSD.

Patch by Viktor Kutuzov!


git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@203099 91177308-0d34-0410-b5e6-96231b3b80d8