C: unconst warnings::Bits strings with -O3

They are written to when warning masks change when using warnings, and
caused segfaults. See issue 340. E.g. t/CORE/op/undef.t with -O3 and many more
CORE tests. This error came up when we added proper INC deps in inc-i340-CC, where
warnings were statically compiled in

Fixed in core:
    t/C-COMPILED/CORE--mro--next_edgecases.t
      Failed tests:  17-18, 22
    t/C-COMPILED/CORE--op--inccode.t
      Failed tests:  17-18, 22
    t/C-COMPILED/CORE--op--length.t
      Failed tests:  17-18, 22
    t/C-COMPILED/CORE--op--sigdispatch.t
      Failed tests:  17-18, 22
    t/C-COMPILED/CORE--op--undef.t
      Failed tests:  17-18, 22
    t/C-COMPILED/CORE--re--charset.t
      Failed tests:  17-18, 22
    t/C-COMPILED/CORE--re--substr.t
      Failed tests:  17-18, 22

Remaing core regressions:
    t/C-COMPILED/CORE--op--closure.t
      Failed tests:  10-11, 20-21
    t/C-COMPILED/CORE--op--dbm.t
      Failed tests:  10-11, 20-21