* 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
