syscall mach:50 leads to valgrind error (using `tzset` causes it):


```c
#include <time.h>
int main (int argc, char *argv[]) {
  tzset();
  return 0;
}
```

```
clang -o /tmp/D20181107T193702   -g $timn_D/bugs/all/t0091.c
valgrind --tool=callgrind /tmp/D20181107T193702
```

```
==19767== Callgrind, a call-graph generating cache profiler
==19767== Copyright (C) 2002-2017, and GNU GPL'd, by Josef Weidendorfer et al.
==19767== Using Valgrind-3.14.0.GIT and LibVEX; rerun with -h for copyright info
==19767== Command: /tmp/D20181107T193702
==19767==
==19767== For interactive control, run 'callgrind_control -h'.
--19767-- UNKNOWN fcntl 98!
--19767-- UNKNOWN fcntl 98! (repeated 2 times)
--19767-- UNKNOWN fcntl 98! (repeated 4 times)
--19767-- UNKNOWN fcntl 98! (repeated 8 times)
--19767-- UNKNOWN fcntl 98! (repeated 16 times)
--19767-- UNKNOWN fcntl 98! (repeated 32 times)
--19767-- WARNING: unhandled amd64-darwin syscall: mach:50
--19767-- You may be able to write your own handler.
--19767-- Read the file README_MISSING_SYSCALL_OR_IOCTL.
--19767-- Nevertheless we consider this a bug.  Please report
--19767-- it at http://valgrind.org/support/bug_reports.html.
eq_SyscallStatus:
  {78 0 43}
  {78 0 40}

valgrind: m_syswrap/syswrap-main.c:438 (Bool eq_SyscallStatus(UInt, SyscallStatus *, SyscallStatus *)): the 'impossible' happened.

host stacktrace:
==19767==    at 0x258024285: ???
==19767==    by 0x2580245F4: ???
==19767==    by 0x2580245D7: ???
==19767==    by 0x25809F774: ???
==19767==    by 0x25809EC33: ???
==19767==    by 0x25809D11A: ???
==19767==    by 0x25809B938: ???
==19767==    by 0x2580AC2CF: ???

sched status:
  running_tid=1

Thread 1: status = VgTs_Runnable (lwpid 771)
==19767==    at 0x1004F5D02: thread_get_special_reply_port (in /usr/lib/system/libsystem_kernel.dylib)
==19767==    by 0x1005A3AB9: xpc_pipe_routine (in /usr/lib/system/libxpc.dylib)
==19767==    by 0x1005A38C4: _xpc_interface_routine (in /usr/lib/system/libxpc.dylib)
==19767==    by 0x1005A34A3: bootstrap_look_up3 (in /usr/lib/system/libxpc.dylib)
==19767==    by 0x1005A32FC: bootstrap_look_up2 (in /usr/lib/system/libxpc.dylib)
==19767==    by 0x1004DA1B4: _notify_fork_child (in /usr/lib/system/libsystem_notify.dylib)
==19767==    by 0x1001E0DCE: _dispatch_client_callout (in /usr/lib/system/libdispatch.dylib)
==19767==    by 0x1001E2514: _dispatch_once_callout (in /usr/lib/system/libdispatch.dylib)
==19767==    by 0x1004DA181: _notify_fork_child (in /usr/lib/system/libsystem_notify.dylib)
==19767==    by 0x1004D8BC2: notify_set_state (in /usr/lib/system/libsystem_notify.dylib)
==19767==    by 0x1004D6977: notify_register_check (in /usr/lib/system/libsystem_notify.dylib)
==19767==    by 0x10033F0E6: notify_register_tz (in /usr/lib/system/libsystem_c.dylib)
==19767==    by 0x10033EA68: tzsetwall_basic (in /usr/lib/system/libsystem_c.dylib)
==19767==    by 0x100000F8A: main (t0091.c:7)

```


## note
that's the root cause of https://github.com/nim-lang/Nim/issues/10121#issuecomment-450453369

I'm on OSX, 10.14.1 (18B75)
valgrind --version
valgrind-3.14.0.GIT

which valgrind
/Users/timothee/homebrew//bin/valgrind