[Bug c/63760] Support __func__ in PROGMEM

[pcdwarf at pcdwarf dot net]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63760

--- Comment #6 from Pierre Colombier <pcdwarf at pcdwarf dot net> ---
by the way a true "immediate constant" version of __func__ just like __FILE__
would be cool, even for non-avr platforms.

I know this is the old behaviour of __FUNCTION__ and that this has been changed
long ago for... well... not-so-trivial reasons...