unix: overwrite _PYTHON_HOST_PLATFORM in macOS cross builds

See the inline comment in the patch. The environment variable as
defined by configure out of the box is not sufficient for macOS because
Python normalizes the value at run-time. This was resulting in incorrect
platform values being used during cross builds.

cpython-unix/build-cpython.sh

@@ -86,7 +86,7 @@ pushd Python-${PYTHON_VERSION}
 # configure doesn't support cross-compiling on Apple. Teach it.
 patch -p1 << "EOF"
 diff --git a/configure b/configure
-index 2d379feb4b..3eb8dbe9ea 100755
+index 1252335472..6665645839 100755
 --- a/configure
 +++ b/configure
 @@ -3301,6 +3301,15 @@ then
@@ -125,7 +125,31 @@ index 2d379feb4b..3eb8dbe9ea 100755
  	*-*-vxworks*)
  		_host_cpu=$host_cpu
  		;;
-@@ -5968,7 +5996,7 @@ $as_echo "#define Py_ENABLE_SHARED 1" >>confdefs.h
+@@ -3359,7 +3381,22 @@ if test "$cross_compiling" = yes; then
+ 		MACHDEP="unknown"
+ 		as_fn_error $? "cross build not supported for $host" "$LINENO" 5
+ 	esac
+-	_PYTHON_HOST_PLATFORM="$MACHDEP${_host_cpu:+-$_host_cpu}"
++
++	case "$host" in
++	  # The _PYTHON_HOST_PLATFORM environment variable is used to
++	  # override the platform name in distutils and sysconfig when
++	  # cross-compiling. On Apple, the platform name expansion logic
++	  # is non-trivial, including renaming MACHDEP=darwin to macosx
++	  # and including the deployment target (or current OS version if
++	  # not set). Our hack here is not generic, but gets the job done
++	  # for python-build-standalone's cross-compile use cases.
++	  aarch64-apple-darwin*)
++	    _PYTHON_HOST_PLATFORM="macosx-${MACOSX_DEPLOYMENT_TARGET}-arm64"
++	    ;;
++	  *)
++	    _PYTHON_HOST_PLATFORM="$MACHDEP${_host_cpu:+-$_host_cpu}"
++	esac
++
+ fi
+ 
+ # Some systems cannot stand _XOPEN_SOURCE being defined at all; they
+@@ -5968,7 +6005,7 @@ $as_echo "#define Py_ENABLE_SHARED 1" >>confdefs.h
  	  BLDLIBRARY='-Wl,+b,$(LIBDIR) -L. -lpython$(LDVERSION)'
  	  RUNSHARED=SHLIB_PATH=`pwd`${SHLIB_PATH:+:${SHLIB_PATH}}
  	  ;;
@@ -134,7 +158,7 @@ index 2d379feb4b..3eb8dbe9ea 100755
      	LDLIBRARY='libpython$(LDVERSION).dylib'
  	BLDLIBRARY='-L. -lpython$(LDVERSION)'
  	RUNSHARED=DYLD_LIBRARY_PATH=`pwd`${DYLD_LIBRARY_PATH:+:${DYLD_LIBRARY_PATH}}
-@@ -6205,16 +6233,6 @@ esac
+@@ -6205,16 +6242,6 @@ esac
    fi
  fi
  
@@ -151,7 +175,7 @@ index 2d379feb4b..3eb8dbe9ea 100755
  case $MACHDEP in
  hp*|HP*)
  	# install -d does not work on HP-UX
-@@ -9541,6 +9559,11 @@ then
+@@ -9541,6 +9568,11 @@ then
  			BLDSHARED="$LDSHARED"
  		fi
  		;;