macOS: use iconv from Homebrew if needed and present

macOS: use iconv from Homebrew if needed and present

The library function iconv(3) supplied with macOS versions 15.7.2
(Sequoia) and 26.1 (Tahoe) is unreliable when doing conversions from
ISO-2022-JP to UTF-8 in multiple steps; t3900 reports this breakage:

  not ok 17 - ISO-2022-JP should be shown in UTF-8 now
  not ok 25 - ISO-2022-JP should be shown in UTF-8 now
  not ok 38 - commit --fixup into ISO-2022-JP from UTF-8

As a workaround, use libiconv from Homebrew, if available.  Search it in
its default locations: /opt/homebrew for Apple Silicon and /usr/local
for macOS Intel, with the former taking precedence.  Respect ICONVDIR if
already set by the user, though.

Helped-by: Koji Nakamaru <koji.nakamaru@gree.net>
Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>