fix #16752: threadvar now works with importcpp types; osx now uses native TLS (--tlsEmulation:off), which can be orders of magnitude faster (#16750)

* osx now uses native TLS, which can be orders of magnitude faster

* add {.cppNonPod.}

* improve test

* changelog, docs, disable part of windows test

tests/misc/mtlsemulation.h

@@ -0,0 +1,37 @@
+#include <stdio.h>
+
+struct Foo1 {
+  /*
+  uncommenting would give:
+  error: initializer for thread-local variable must be a constant expression
+  N_LIB_PRIVATE NIM_THREADVAR Foo1 g1__9brEZhPEldbVrNpdRGmWESA;
+  */
+  // Foo1() noexcept { }
+
+  /*
+  uncommenting would give:
+  error: type of thread-local variable has non-trivial destruction
+  */
+  // ~Foo1() { }
+  int x;
+};
+
+struct Foo2 {
+  Foo2() noexcept { }
+  ~Foo2() { }
+  int x;
+};
+
+static int ctorCalls = 0;
+static int dtorCalls = 0;
+
+struct Foo3 {
+  Foo3() noexcept {
+    ctorCalls = ctorCalls + 1;
+    x = 10;
+  }
+  ~Foo3() {
+    dtorCalls = dtorCalls + 1;
+  }
+  int x;
+};