zorg Xcode python test suite target arch update

This changes the Xcode target used by the Green Dragon Xcode CI.
When calling xcodebuild with LLDB_PYTHON_TESTSUITE_ARCH set, the
arch's xUnit XML output is now set to an arch-specific filename:

$(BUILD_DIR)/test-results-$(LLDB_PYTHON_TESTSUITE_ARCH).xml.

The change also ensures that the Python testsuite sees the Xcode
build settings passed in through environment variables.

llvm-svn: 282605