Make FML_LOG safe from static initialization (flutter#42219)

I ran into this while trying to get some printing going for places where we're creating thread local keys. 

Supposedly, just including `<iostream>` should statically initialize `std::cout/cerr`, but it gets really hard to reason about whether your statically initialized code is going to be initialized before or after that happens. I tried making sure that the TU for `fml/logging.cc` did that initialization statically, but that also failed in the verison of the test included here (it passed in some other iterations that modified run_all_unittests.cc). We _could_ make sure it happens each and every time we touch `std::cerr` but ... we could also just use `fprintf(stderr, ...)` and it works just fine.

/cc @flar who ran into problems around this a little while back and was asking about it.

fml/logging.cc

@@ -123,8 +123,9 @@ LogMessage::~LogMessage() {
   fx_logger_log_with_source(fx_log_get_logger(), fx_severity, nullptr, file_,
                             line_, stream_.str().c_str());
 #else
-  std::cerr << stream_.str();
-  std::cerr.flush();
+  // Don't use std::cerr here, because it may not be initialized properly yet.
+  fprintf(stderr, "%s", stream_.str().c_str());
+  fflush(stderr);
 #endif
 
   if (severity_ >= LOG_FATAL) {

fml/logging_unittests.cc

@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+#include <signal.h>
+
 #include "flutter/fml/build_config.h"
 #include "flutter/fml/log_settings.h"
 #include "flutter/fml/logging.h"
@@ -19,6 +21,41 @@
 namespace fml {
 namespace testing {
 
+#ifndef OS_FUCHSIA
+class MakeSureFmlLogDoesNotSegfaultWhenStaticallyCalled {
+ public:
+  MakeSureFmlLogDoesNotSegfaultWhenStaticallyCalled() {
+    SegfaultCatcher catcher;
+    // If this line causes a segfault, FML is using a method of logging that is
+    // not safe from static initialization on your platform.
+    FML_LOG(INFO)
+        << "This log exists to verify that static logging from FML works.";
+  }
+
+ private:
+  struct SegfaultCatcher {
+    typedef void (*sighandler_t)(int);
+
+    SegfaultCatcher() {
+      handler = ::signal(SIGSEGV, SegfaultHandler);
+      FML_CHECK(handler != SIG_ERR);
+    }
+
+    ~SegfaultCatcher() { FML_CHECK(::signal(SIGSEGV, handler) != SIG_ERR); }
+
+    static void SegfaultHandler(int signal) {
+      fprintf(stderr,
+              "FML failed to handle logging from static initialization.\n");
+      exit(signal);
+    }
+
+    sighandler_t handler;
+  };
+};
+
+static MakeSureFmlLogDoesNotSegfaultWhenStaticallyCalled fml_log_static_check_;
+#endif  // !defined(OS_FUCHSIA)
+
 int UnreachableScopeWithoutReturnDoesNotMakeCompilerMad() {
   KillProcess();
   // return 0; <--- Missing but compiler is fine.