configure: don't enable ppc64abi32-linux-user by default

The user can still enable this explicitly but they will get a warning
at the end of configure for their troubles. This also drops any builds
of ppc64abi32 from our CI tests.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20200909112742.25730-7-alex.bennee@linaro.org>