tests/docker: Fix fedora-i386-cross cross-compilation

tests/docker: Fix fedora-i386-cross cross-compilation

By using PKG_CONFIG_PATH instead of PKG_CONFIG_LIBDIR,
we were still including the 64-bit packages.  Install
pcre-devel.i686 to fill a missing glib2 dependency.

By using --extra-cflags instead of --cpu, we incorrectly
use the wrong probing during meson.

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Richard W.M. Jones <rjones@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Message-Id: <20210930163636.721311-3-richard.henderson@linaro.org>