build-many-glibcs.py: Add a s390x -O3 glibc variant.

build-many-glibcs.py: Add a s390x -O3 glibc variant.

On s390x, gcc inlines more aggresive compared to other architectures.
This occaisionally leads to build warnings / errors.

Therefore this patch adds a s390x glibc variant with optimization.

There is the ccopts field which contain ABI options which are passed
to configure as CC / CXX.  Now there is also the cflags field which
contains non-ABI options like -g or -O. Those are passed to configure
as CFLAGS / CXXFLAGS.

Currently CC is passed to conformtest.py or linknamespace.py but not
the CFLAGS.