powerpc/64s/exception: Add the virt variant of the denorm interrupt handler

All other virt handlers have the prolog code in the virt vector rather
than branch to the real vector. Follow this pattern in the denorm virt
handler.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20190802105709.27696-25-npiggin@gmail.com