PR libstdc++/87642 handle multibyte thousands separators from libc

PR libstdc++/87642 handle multibyte thousands separators from libc

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