Fix _pthread_init illegal instruction by providing a value for ptr_munge

through the PTHREAD_PTR_MUNGE_TOKEN environment variable (dummy value)

coregrind/m_initimg/initimg-darwin.c

@@ -165,7 +165,11 @@ static HChar** setup_client_env ( HChar** origenv, const HChar* toolname)
 
    /* Allocate a new space */
    ret = VG_(malloc) ("initimg-darwin.sce.3", 
+#if DARWIN_VERS >= DARWIN_10_15
+                      sizeof(HChar *) * (envc+3+1)); /* 3 new entries + NULL */
+#else
                       sizeof(HChar *) * (envc+2+1)); /* 2 new entries + NULL */
+#endif
 
    /* copy it over */
    for (cpp = ret; *origenv; )
@@ -216,6 +220,10 @@ static HChar** setup_client_env ( HChar** origenv, const HChar* toolname)
 
       ret[envc++] = cp;
    }
+#if DARWIN_VERS >= DARWIN_10_15
+   // pthread really wants a non-zero value for ptr_munge
+   ret[envc++] = "PTHREAD_PTR_MUNGE_TOKEN=0x00000001";
+#endif
 
 
    /* ret[0 .. envc-1] is live now. */