Add back non-const abc::to_str_backend char{8,16,32}_t [] specialization
Add back non-const abc::to_str_backend char{8,16,32}_t [] specialization

Commit 04d9cc80f3142db7cd235f4a136b874f6215b73d changed these
specializations to a const template type, but GCC does not like this
change.

Provide both specializations (const [] and non-const []) until I
understand how to avoid one of them.

raffaellod/lofty,raffaellod/lofty