[libcxx] Construct __start_std_streams at high init priority

[ldionne]

Summary

This allows normal static constructors use use std::cout, etc.

We've been patching this in emscripten for a long time now.
See https://github.com/emscripten-core/emscripten/issues/3824

Fixes: PR28954

[sbc100]

See https://github.com/emscripten-core/emscripten/issues/3824 for a simple repro

[sbc100]

Oops, I saw you already have https://reviews.llvm.org/D31413 out .. for a while now. And the associated bug: http://llvm.org/PR28954

[ldionne]

Also, please coordinate with @EricWF on https://reviews.llvm.org/D31413. I haven't followed the whole thing, but what are we blocked on? It seems like a fairly serious issue that's worth fixing.

libcxx/src/iostream.cpp

83

Building the dylib on Apple isn't supported with GCC. It may happen to work, but that's not maintained. Please don't add a code path to support it -- it just makes things slightly more complex while giving a false impression of support.

[sbc100]

This came up again in wasi-sdk: https://github.com/WebAssembly/wasi-sdk/issues/153

[ldionne]

Let's close this in favour of https://reviews.llvm.org/D31413 to clean things up.

[ldionne]

This is abandoned for https://reviews.llvm.org/D31413.

libcxx/src/iostream.cpp

 #endif
 ;
 _ALIGNAS_TYPE (wostream) _LIBCPP_FUNC_VIS char wclog[sizeof(wostream)]
 #if defined(_LIBCPP_ABI_MICROSOFT) && defined(__clang__)
 __asm__("?wclog@" _LIBCPP_ABI_NAMESPACE_STR "@std@@3V?$basic_ostream@_WU?$char_traits@_W@" _LIBCPP_ABI_NAMESPACE_STR "@std@@@12@A")
 #endif
 ;
 
-_LIBCPP_HIDDEN ios_base::Init __start_std_streams;
+// Detect if the init_priority attribute is supported.
+#if (defined(_LIBCPP_COMPILER_GCC) && defined(__APPLE__)) \
+  || defined(_LIBCPP_COMPILER_MSVC)
+// GCC on Apple doesn't support the init priority attribute,
+// and MSVC doesn't support any GCC attributes.
+# define _LIBCPP_INIT_PRIORITY_MAX
+#else
+# define _LIBCPP_INIT_PRIORITY_MAX __attribute__((init_priority(101)))
+#endif
+
+_LIBCPP_HIDDEN ios_base::Init __start_std_streams _LIBCPP_INIT_PRIORITY_MAX;
 
 // On Windows the TLS storage for locales needs to be initialized before we create
 // the standard streams, otherwise it may not be alive during program termination
 // when we flush the streams.
 static void force_locale_initialization() {
 #if defined(_LIBCPP_MSVCRT_LIKE)
   static bool once = []() {
     auto loc = newlocale(LC_ALL_MASK, "C", 0);