package/pkg-python: fix get_platform for target builds

Replicate the logic used to set _PYTHON_HOST_PLATFORM in configure.ac:
https://github.com/python/cpython/blob/v3.10.2/configure.ac#L441-L446

This should correctly set the sysconfig platform for cross builds:
https://github.com/python/cpython/blob/v3.10.2/Lib/sysconfig.py#L704-L706

Not setting this appears to cause distutils/setuptools to use the
wrong build directory names for c extensions, however this doesn't
seem to cause build or runtime errors as the installation paths still
appear to be correct for staging/target installs.

Signed-off-by: James Hilliard <james.hilliard1@gmail.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>

package/pkg-python.mk

@@ -20,6 +20,13 @@
 #
 ################################################################################
 
+ifeq ($(BR2_arm)$(BR2_armeb),y)
+PKG_PYTHON_ARCH = arm
+else
+PKG_PYTHON_ARCH = $(ARCH)
+endif
+PKG_PYTHON_HOST_PLATFORM = linux-$(PKG_PYTHON_ARCH)
+
 # basename does not evaluate if a file exists, so we must check to ensure
 # the _sysconfigdata__linux_*.py file exists. The "|| true" is added to return
 # an empty string if the file does not exist.
@@ -33,6 +40,7 @@ PKG_PYTHON_DISTUTILS_ENV = \
 	LDSHARED="$(TARGET_CROSS)gcc -shared" \
 	PYTHONPATH="$(if $(BR2_PACKAGE_PYTHON3),$(PYTHON3_PATH),$(PYTHON_PATH))" \
 	PYTHONNOUSERSITE=1 \
+	_PYTHON_HOST_PLATFORM="$(PKG_PYTHON_HOST_PLATFORM)" \
 	_PYTHON_PROJECT_BASE="$(if $(BR2_PACKAGE_PYTHON3),$(PYTHON3_DIR),$(PYTHON_DIR))" \
 	_PYTHON_SYSCONFIGDATA_NAME="$(PKG_PYTHON_SYSCONFIGDATA_NAME)" \
 	_python_sysroot=$(STAGING_DIR) \
@@ -61,6 +69,7 @@ HOST_PKG_PYTHON_DISTUTILS_INSTALL_OPTS = \
 
 # Target setuptools-based packages
 PKG_PYTHON_SETUPTOOLS_ENV = \
+	_PYTHON_HOST_PLATFORM="$(PKG_PYTHON_HOST_PLATFORM)" \
 	_PYTHON_PROJECT_BASE="$(if $(BR2_PACKAGE_PYTHON3),$(PYTHON3_DIR),$(PYTHON_DIR))" \
 	_PYTHON_SYSCONFIGDATA_NAME="$(PKG_PYTHON_SYSCONFIGDATA_NAME)" \
 	PATH=$(BR_PATH) \