Move the flds instruction emulation failure handling code to a header
so it can be re-used in an upcoming test.

No functional change intended.
Signed-off-by: David Matlack <dmatlack@google.com>
Link: https://lore.kernel.org/r/20221102184654.282799-5-dmatlack@google.comSigned-off-by: Sean Christopherson <seanjc@google.com>