18763: sys/tiny_strerror: add missing error codes r=benpicco a=maribu
### Contribution description
When double-checking the error codes provided by newlib by default (without magic defines, such as `__LINUX_ERRNO_EXTENSIONS__` or `__CYGWIN__`), some where still missing in `tiny_strerror()`. This adds the missing ones.
This in turn showed that three errno codes were missing in the avr-libc compat `errno.h`, which are added as well.
### Testing procedure
Murdock should double check that the added errno codes indeed are defined by default.
### Issues/PRs references
None
Co-authored-by: Marian Buschsieweke <marian.buschsieweke@ovgu.de>
Add a minimal implementation of `strerror()`.
All other targets use either newlib or picolibc and provide this
function, add it to AVR so we can rely on it in common code.
Split atmega_common code into avr8_common folder. This moves common
avr8 code to be used for all avr8 variants: tiny, mega and xmega.
Signed-off-by: Gerson Fernando Budke <nandojve@gmail.com>
