After looking more in depth, gdbserver must not be terminated
in PRE(posix_spawn) on MacOS: this is running in the parent and
(on MacOS) is a single syscall similar to a fork+exec.
On linux, posix_spawn is implemented using 2 syscalls
(fork followed by exec).
git-svn-id: svn://svn.valgrind.org/valgrind/trunk@12882