Allow Print::println() to work with PROGMEM strings

[earlephilhower]

Adjust the ::write implementation in Print and its overridden copy in
UART to allow it to silentely accept PROGMEM strings (since there is no
write_P macro).

Fixes #6383

[dontsovcmc]

Thanks a lot!
Is it possible to show compile error if user call function that doesn't support PROGMEM array?
I mean if user writes const char text[] PROGMEM = "some text"; and calls f(text);
that has string actions inside.

[d-a-v]

There is no compilation error in that case.
But a runtime crash may/will happen.