python3: Use default _PYTHON_HOST_PLATFORM

python3: Use default _PYTHON_HOST_PLATFORM

This lets the Python build process set _PYTHON_HOST_PLATFORM instead of
forcing an explicit value.

Also:

* Save the target _PYTHON_HOST_PLATFORM value during Build/InstallDev
  for use when building target Python packages (in python3-package.mk).

* Use the (mostly) default PYTHON_FOR_BUILD value, instead patch
  configure to remove the platform triplet from the sysconfigdata file
  name.

* Remove the "CROSS_COMPILE=yes" make variable (there is no indication
  that this variable is necessary).

* Force host pip to build packages from source instead of downloading
  binary wheels.

  Previously, host pip can download universal (platform-independent)
  wheels but not platform-specific wheels, because of the custom
  _PYTHON_HOST_PLATFORM value. (Packages that do not have universal
  wheels would be compiled from source.)

  With a correct _PYTHON_HOST_PLATFORM, host pip can install
  platform-specific wheels as well. However, the pre-built shared object
  (.so) files in these wheels will have the host's platform triplet in
  their file names. When target Python packages are built (using the
  target's _PYTHON_HOST_PLATFORM), Python will not use these shared
  object files.

  By forcing host pip to build packages from source, the built shared
  object files will not have the platform triplet in their file names.
  (Host Python has been patched to remove the platform triplet from file
  names.) This allows these packages to be used when building target
  Python packages.

  (The net effect of this complete change is that platform-dependent
  packages will continue to be compiled from source, while
  platform-independent packages will now also be compiled from source.)

Signed-off-by: Jeffery To <jeffery.to@gmail.com>