MAINTAINERS: scripts to generate the file from yaml data

Convert the existing MAINTAINERS data to YAML and add a script to
rebuild the MAINTAINERS file from it.  Rebuild the MAINTAINERS file from
that data.

Update the testsuite to check that the MAINTAINERS file is correctly
generated from the YAML.

Remove contrib/check-MAINTAINERS.py: it's nolonger needed now that
we generate the file automatically.

ChangeLog:

	* MAINTAINERS.yml: New file.
	* MAINTAINERS: Regenerate from MAINTAINERS.yml.

contrib/ChangeLog:

	* gen-MAINTAINERS.py: New file.
	* maintainer_utils.py: New file.
	* check-MAINTAINERS.py: Delete.

gcc/testsuite/ChangeLog:

	* gcc.src/maintainers.exp: