Fix iostream related issues (#5047)

* add stubs for more exception throw calls

Fixes https://github.com/esp8266/Arduino/issues/3358

* libc: make putc_r implementation weak

newlib provides its own implementation of _putc_r, which will call
_write_r (possibly after buffering). Make our implementation weak to
allow using the one from newlib.

Fixes https://github.com/esp8266/Arduino/issues/4630

* libc: fix incorrect return value of _write_r call

Should return number of bytes written, actually returned zero. This
resulted in std::cout going into failed state after the first write.

* tests: add test for output to std::cout

tests/device/test_iostream/test_iostream.ino

@@ -0,0 +1,34 @@
+#include <ESP8266WiFi.h>
+#include <BSTest.h>
+#include <sstream>
+#include <iostream>
+
+BS_ENV_DECLARE();
+
+void setup()
+{
+    Serial.begin(115200);
+    BS_RUN(Serial);
+}
+
+TEST_CASE("can print to std::cout", "[iostream]")
+{
+    std::stringstream test_stream("");
+    test_stream << "hello stream";
+
+    // empty the RX buffer, just in case
+    Serial.readString();
+
+    USC0(0) |= (1 << UCLBE);  // enable loopback
+    std::cout << test_stream.str().c_str() << std::endl;
+    delay(100);
+    USC0(0) &= ~(1 << UCLBE);  // disable loopback
+    
+    String result = Serial.readStringUntil('\n');
+
+    Serial.printf("result: '%s'\n", result.c_str());
+
+    CHECK(result == test_stream.str().c_str());
+}
+
+void loop() { }