Unify OS X/POSIX/Linux Host spawn support.

This fixes a bug under Linux where spawning a process via
Host::LaunchProcess was disabling all blockable signals on the
launched process.  This caused strange behavior when attempting
to kill the lldb-gdbserver process, as the child generally would
not die unless killed with a non-blockable signal (e.g. 'kill -9').

This change moves several functions out of macosx/Host.mm into
common/Host.cpp.  In addition, two functions that needed to work
across common/Host.cpp and macosx/Host.mm were moved into the Host.h
header file.