Now that selftests support printf() in the guest, report unexpected
exceptions via the regular assertion framework.  Exceptions were special
cased purely to provide a better error message.  Convert only x86 for now,
as it's low-hanging fruit (already formats the assertion in the guest),
and converting x86 will allow adding asserts in x86 library code without
needing to update multiple tests.

Once all other architectures are converted, this will allow moving the
reporting to common code, which will in turn allow adding asserts in
common library code, and will also allow removing UCALL_UNHANDLED.

Link: https://lore.kernel.org/r/20240719235107.3023592-6-seanjc@google.comSigned-off-by: Sean Christopherson <seanjc@google.com>