Split basic functionality gn.bbclass into gn_base.bbclass

This provides a gn_base.bbclass which can be used instead of gn.bbclass in
recipes where a different method for managing toolchain configuration is used.

Note, any recipes calling gn_do_configure or gn_do_compile will need to call the
new gn_base_do_configure and gn_base_do_compile respectively instead.

Signed-off-by: Esben Haabendal <esben.haabendal@huawei.com>