libcgroup: upgrade to 2.0

libcgroup: upgrade to 2.0

Our UPSTREAM_CHECK_URI was looking at SourceForge, but development has
moved to GitHub so update the URI.

Update to 2.0.

Swap musl-decls-compat.patch for a backport of an upstream commit.

Replace do_install commands with a backport of an upstream commit to
install the PAM module correctly.

Don't mess about installing the library into base_libdir as the /lib vs
/usr/lib prefix split is moot these days.

Delete libcgroupfortesting.so as we don't install the test suite.

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