osxkeychain: define build targets in the top-level Makefile.

osxkeychain: define build targets in the top-level Makefile.

The fix for git-credential-osxkeychain in 4580bcd235 (osxkeychain: avoid
incorrectly skipping store operation, 2025-11-14) introduced linkage
with libgit.a, and its Makefile was adjusted accordingly. However, the
build fails as of 864f55e190 because several macOS-specific refinements
were applied to the top-level Makefile and config.mak.uname, such as:

  - 363837afe7 (macOS: make Homebrew use configurable, 2025-12-24)
  - cee341e9dd (macOS: use iconv from Homebrew if needed and present,
    2025-12-24)
  - d281241518 (utf8.c: enable workaround for iconv under macOS 14/15,
    2026-01-12)

Since libgit.a and its corresponding header files depend on many flags
defined in the top-level Makefile, these flags must be consistently
defined when building git-credential-osxkeychain. Continuing to manually
adjust the git-credential-osxkeychain Makefile is cumbersome and
fragile.

Define the build targets for git-credential-osxkeychain in the top-level
Makefile and modify its local Makefile to simply rely on those targets.

Helped-by: Junio C Hamano <gitster@pobox.com>
Reported-by: D. Ben Knoble <ben.knoble@gmail.com>
Helped-by: Kristoffer Haugsbakk <kristofferhaugsbakk@fastmail.com>
Signed-off-by: Koji Nakamaru <koji.nakamaru@gree.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>