This adds a register in the middle of the countzero computation,
so that we now have two cycles to count leading or trailing zeroes
instead of just one. Execute1 now outputs a one-cycle stall signal
when it encounters a cntlz* or cnttz* instruction. With this,
the countzero path no longer fails timing on the Artix-7 at 100MHz.
Signed-off-by: Paul Mackerras <email@example.com>