gh-114099: Refactor configure and Makefile to accomodate non-macOS frameworks (#115120)

Part of the PEP 730 work to add iOS support.

This change lays the groundwork for introducing iOS/tvOS/watchOS 
frameworks; it includes the structural refactoring needed so that iOS
branches can be added into in a subsequent PR.

Summary of changes:
* Updates config.sub to the 2024-01-01 release. This is the "as 
  released" version of config.sub.
* Adds a RESSRCDIR variable to allow sharing of macOS and iOS Makefile 
  steps.
* Adds an INSTALLTARGETS variable so platforms can customise which
  targets are actually installed. This will be used to exclude certain
  targets (e.g., binaries, manfiles) from iOS framework installs.
* Adds a PYTHONFRAMEWORKINSTALLNAMEPREFIX variable; this is used as
  the install name for the library. This is needed to allow for iOS
  frameworks to specify an @rpath-based install name.
* Evaluates MACHDEP earlier in the configure process so that
  ac_sys_system is available.
* Modifies _PYTHON_HOST_PLATFORM evaluation for cross-platform builds
  so that the CPU architecture is differentiated from the host
  identifier. This will be used to generate a _PYTHON_HOST_PLATFORM
  definition that includes ABI information, not just CPU architecture.
* Differentiates between SOABI_PLATFORM and PLATFORM_TRIPLET.
  SOABI_PLATFORM is used in binary module names, and includes the ABI,
  but not the OS or CPU architecture (e.g.,
  math.cpython-313-iphonesimulator.dylib). PLATFORM_TRIPLET is used
  as the sys._multiarch value, and on iOS will contains the ABI and
  architecture (e.g., iphoneos-arm64). This differentiation hasn't
  historically been needed because while macOS is a multiarch platform,
  it uses a bare darwin as PLATFORM_TRIPLE.
* Removes the use of the deprecated -Wl,-single_module flag when
  compiling macOS frameworks.
* Some whitespace normalisation where there was a mix of spaces and tabs 
  in a single block.