Implement missing PROGMEM / _P functions in pgmspace.h

[matthijskooijman]

Pgmspace.h provides implementations for functions like memcpy_P, which is the PROGMEM version of memcpy. On AVR, these functions handle loading data from flash (PROGMEM) into RAM before operating on it, on other platforms and in arduino_ci, these are just no-ops wrapper functions.

In #144, these wrappers were changed from macros to inline functions. However, for some of the less common functions (like memccpy), the to-be-wrapped function (without the _P) was not available when compiling under CYGWIN. It was not clear why exactly, since the header files they were defined in did seem to contain the proper declarations and the needed macros for them also seemed set. To prevent stalling merging that PR, these problematic function wrappers were just disabled in pgmspace.h, but it would be good to reintroduce (uncomment) them and make them work on cygwin too.

[ianfixes]

Relates to the string TODO: These functions cannot be found on the CYGWIN test build in the codebase