Don’t set PadlistMAXNAMED for single-char entries

This fixes perl #123430.

Single-character entries like "$" and "&" are not actually named
entries, but are partially treated as such for the sake of bookkeeping
and scope.  (E.g., a flipflop target must have the same lifetime as a
state variable.)

PadlistMAXNAMED is an optimisation that marks the offset of the high-
est pad slot with a name.  If there any many anonymous pad slots after
the last named one, we don’t want to search through them when looking
up a symbol.  So we mark the maximum named slot and skip half the pad
if we are lucky.

Commit v5.21.4-51-g14d9114 caused flipflop targets to be allocated as
variables named "$", causing compilation of some generated code to
slow down.  At compile time, the name pad is not extended to the
length of the pad until the end of subroutine compilation.  So prior
to 14d9114 flipflop targets would not make the name pad any longer.
Now that flipflop targets expand the name pad, stop setting
PadlistMAXNAMED, so that things are no slower than before.

This is not really the best fix, IMO, because code that is sensitive
to this will slow down dramatically if you tweak it ever so slightly
by adding a ‘my $x’ here or there.