Test failures with musl libc

Description

Follow-up to https://github.com/php/php-src/issues/13696. Musl is partially tested on the CI, but the Alpine job installs gnu-libiconv-dev which hides the following errors. With the latest php-8.5.7 release:

=====================================================================
FAILED TEST SUMMARY
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Bug #48147 (iconv with //IGNORE cuts the string) [ext/iconv/tests/bug48147.phpt]
Bug #52211 (iconv() returns part of string on error) [ext/iconv/tests/bug52211.phpt]
Bug #76249 (stream filter convert.iconv leads to infinite loop on invalid sequence) [ext/iconv/tests/bug76249.phpt]
EUC-JP to ISO-2022-JP [ext/iconv/tests/eucjp2iso2022jp.phpt]
iconv_mime_encode() [ext/iconv/tests/iconv_mime_encode.phpt]

I have an old PR at https://github.com/php/php-src/pull/16840 whose details I've mostly forgotten. I can start by cherry-picking the easy parts of that.

PHP Version

PHP 8.5.7

Operating System

Gentoo linux

Beginne mit dem Alpine-CI-Job und den fehlschlagenden Tests in ext/iconv/tests/, insbesondere bug48147.phpt, bug52211.phpt, bug76249.phpt, eucjp2iso2022jp.phpt und iconv_mime_encode.phpt. Vergleiche den Job mit und ohne gnu-libiconv-dev und prüfe anschließend die damit verbundene Arbeit in PR 16840. Die Aufgabe ist abgeschlossen, wenn die aufgeführten Tests gegen musl bestehen, ohne dass dieses Paket die Fehler maskiert.