Fibers need mmap(2) MAP_STACK.

Index: runtime/druntime/src/core/sys/posix/sys/mman.d
--- runtime/druntime/src/core/sys/posix/sys/mman.d.orig
+++ runtime/druntime/src/core/sys/posix/sys/mman.d
@@ -460,6 +460,7 @@ else version (OpenBSD)
     enum MAP_PRIVATE    = 0x0002;
     enum MAP_FIXED      = 0x0010;
     enum MAP_ANON       = 0x1000;
+    enum MAP_STACK      = 0x4000;
 
     enum MAP_FAILED     = cast(void*)-1;
 
