fix TT #1930, optimized ppc: n mod 0.0 failure

we need to use volatile temporaries really with such floats.
Fixes t/op/number.t test 50
[docs] README.pod tell that --optimize is recommended for packagers

that it is 2x faster. Now that all known --optimize bugs are fixed (even on powerpc TT #1930)
[cage] gc_gms.c fix compiler cast warnings I introduced
imcc.l: re-add missing optimizer_level and ITPUREFUNC