pc-bios/s390-ccw: detect CC options just once

In preparation for adding Docker container support, detect compiler options
just once rather than once per Make run; container startup overhead is
substantial and doing the detection just once makes things faster.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20220929114231.583801-15-alex.bennee@linaro.org>