#174 %REP should support e.g. * for infinite loops

[nasm64developer]

If the loop count is not known up front, then
code may wish to enter an infinite loop and
terminate it by means of %EXITREP inside
a suitable conditional block.

One way to achieve this is by accepting "*"
as a valid loop count, setting a boolean flag
for it (abuse e.g. plus), and not decrementing
in_progress if that flag is set. To make it work
two more changes are needed: the if() before
the decrement needs to let flag=true through
and %EXITREP needs to clear the flag.