core.sys.posix.ucontext: Define mcontext_t and ucontext_t for Darwin

Author: ibuclaw

src/core/sys/posix/ucontext.d

@@ -24,6 +24,15 @@ nothrow:
 @nogc:
 @system:
 
+version (OSX)
+    version = Darwin;
+else version (iOS)
+    version = Darwin;
+else version (TVOS)
+    version = Darwin;
+else version (WatchOS)
+    version = Darwin;
+
 version (MIPS32)  version = MIPS_Any;
 version (MIPS64)  version = MIPS_Any;
 version (PPC)     version = PPC_Any;
@@ -910,6 +919,22 @@ else version (CRuntime_Musl)
     else
         static assert(0, "unimplemented");
 }
+else version (Darwin)
+{
+    alias mcontext_t = void;
+
+    struct ucontext
+    {
+        int         uc_onstack;
+        sigset_t    uc_sigmask;
+        stack_t     uc_stack;
+        ucontext*   uc_link;
+        size_t      uc_mcsize;
+        mcontext_t* uc_mcontext;
+    }
+
+    alias ucontext_t = ucontext;
+}
 else version (FreeBSD)
 {
     // <machine/ucontext.h>