x86: Make exception_mnemonic() visible to the tests

exception_mnemonic() is a useful function for more than just desc.c.
Make it global, so it can be used in other KUT tests.

Signed-off-by: Aaron Lewis <aaronlewis@google.com>
Reviewed-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20220125203127.1161838-2-aaronlewis@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>