KVM: selftests: Add a selftest for guest prints and formatted asserts

Add a test to exercise the various features in KVM selftest's local
snprintf() and compare them to LIBC's snprintf() to ensure they behave
the same.

This is not an exhaustive test.  KVM's local snprintf() does not
implement all the features LIBC does, e.g. KVM's local snprintf() does
not support floats or doubles, so testing for those features were
excluded.

Testing was added for the features that are expected to work to
support a minimal version of printf() in the guest.

Signed-off-by: Aaron Lewis <aaronlewis@google.com>
[sean: use UCALL_EXIT_REASON, enable for all architectures]
Link: https://lore.kernel.org/r/20230731203026.1192091-4-seanjc@google.com


Signed-off-by: Sean Christopherson <seanjc@google.com>