syscall: serialize locks on ForkLock on platforms where forkExecPipe is not atomic
In CL 421441, we changed syscall to allow concurrent calls to
forkExec.
On platforms that support the pipe2 syscall that is the right
behavior, because pipe2 atomically opens the pipe with CLOEXEC already
set.
However, on platforms that do not support pipe2 (currently aix and
darwin), syscall.forkExecPipe is not atomic, and the pipes do not
initially have CLOEXEC set. If two calls to forkExec proceed
concurrently, a pipe intended for one child process can be
accidentally inherited by the other. If the process is long-lived, the
pipe can be held open unexpectedly and prevent the parent process from
reaching EOF reading the child's status from the pipe.