If a locale's THOUSANDS_SEP or MON_THOUSANDS_SEP string is not a
single character we either need to narrow it to a single char or
ignore it (and therefore disable digit grouping for that facet).
PR libstdc++/87642
* config/locale/gnu/monetary_members.cc
(moneypunct<char, true>::_M_initialize_moneypunct): Use
__narrow_multibyte_chars to convert multibyte thousands separators
to a single char.
* config/locale/gnu/numeric_members.cc
(numpunct<char>::_M_initialize_numpunct): Likewise.
(__narrow_multibyte_chars): New function.
From-SVN: r265286
