sys/log: make log_write nonstatic for log_color

[kfessel]

Contribution description

If log_color is used: this moves log_write from its header (being implemtented static to its own compile unit making it used accross multiple compile units)

Testing procedure

run tests/log_*

Issues/PRs references

#18379 i was asked by @miri64 to check if it increases the rom size (it didn't) while checking for that I found that log_write seems to not be inlined -> using one for all would decrease the size.

[kfessel]

I prefer https://github.com/RIOT-OS/RIOT/pull/18425 for its simplicity