classes/python_pep517: implement a standard do_compile

classes/python_pep517: implement a standard do_compile

As all PEP517-compliant build systems have a universal API, we can ask
that users of this class set PEP517_BUILD_API to the class that implements
this API and call it ourselves, instead of users needing to implement
near-identical do_compile tasks themselves.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>