iconv regression in recent macOS distributions

The recent macOS Sonoma release distributed an iconv() that introduced regression in the encoding004 test of the base testsuite that tests roundtripping of CP936.

The release seems to have also fixed an issue where LC_ALL=C would be ignored in macOS and Unicode was by default regardless, which caused failures in T2507, T6037, and T8959a.

Unexpected passes:
   /var/folders/0p/k15lrkf913923yn8w6nk3kc80000hm/T/ghctest-32bcsuxp/test   spaces/testsuite/tests/driver/T2507.run   T2507 [unexpected] (normal)
   /var/folders/0p/k15lrkf913923yn8w6nk3kc80000hm/T/ghctest-32bcsuxp/test   spaces/testsuite/tests/driver/T6037.run   T6037 [unexpected] (normal)
   /var/folders/0p/k15lrkf913923yn8w6nk3kc80000hm/T/ghctest-32bcsuxp/test   spaces/testsuite/tests/driver/T8959a.run  T8959a [unexpected] (normal)
Unexpected failures:
   /var/folders/0p/k15lrkf913923yn8w6nk3kc80000hm/T/ghctest-32bcsuxp/test   spaces/libraries/base/tests/IO/encoding004.run  encoding004 [bad exit code (1)] (normal)