MAINTAINERS: Add a general architecture section for x86

MAINTAINERS: Add a general architecture section for x86

It's a little bit weird that the files in target/i386/ which
are not in a subfolder there do not have any associated
maintainer (and thus nobody might be CC:-ed on changes to
these files). We should have a general x86 section for these
files, similar to what we already have for s390x and mips.
Since Paolo is already listed as maintainer for both, the
x86 KVM and TCG CPUs, I'd like to suggest him as maintainer
for the general files, too.

Message-ID: <20230929134551.395438-1-thuth@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>