Fix incomplete commit of http://llvm.org/viewvc/llvm-project?rev=147609&view=rev:

This patch combines common code from Linux and FreeBSD into
a new POSIX platform.  It also contains fixes for 64bit FreeBSD.

The patch is based on changes by Mark Peek <mp@FreeBSD.org> and
"K. Macy" <kmacy@freebsd.org> in their github repo located at
https://github.com/fbsd/lldb.

llvm-svn: 147613