implement iconv from gnu upstream

I tried my best at https://gitlab.alpinelinux.org/alpine/aports/-/merge_requests/61689 but it looks like php-src compile options do not properly work to activate gnu iconv and use "shared" as --with-iconv option.

This ticket is for tracking these efforts also here.

Here is a list of configurations I tried at https://gitlab.alpinelinux.org/alpine/aports/-/merge_requests/61689#note_382099 