Add a bandaid to fix the FreeBSD build

r218568 added an explicit #include of the Linux ProcessMonitor.h to
POSIXThread.cpp, rather than including just "ProcessMonitor.h" and
relying on the build infrastructure for the appropriate paths.

For now add #ifdefs in the source to use the FreeBSD or Linux header
as appropriate; a cleaner fix (and perhaps some refactoring of the
POSIX classes) should still be done later.

llvm-svn: 218762