cpu/avr8_common: add strings.h for compatibility

[maribu]

Contribution description

This adds strings.h missing in AVR libc and implements the most commonly used functions in it (which are all trivial).

Testing procedure

The functions are trivial enough that code review should be sufficient. But I could add a test application, if reviewers would prefer.

Issues/PRs references

Noticed due to Murdock failures in #18621

[maribu]

@benpicco I wonder if it would make sense to cherry pick the commit here into #18621 - I would actually like to add a few lines of unit tests to ensure consistent behavior among C libs, but I rather have that in the test app #18621 introduced.

[maribu]

Thx :)