libstdc++: avoid -Wsign-compare

-Wsign-compare complained about these comparisons between (unsigned) size_t
and (signed) streamsize, or between (unsigned) native_handle_type
and (signed) -1.  Fixed by adding casts to unify the types.

libstdc++-v3/ChangeLog:

	* include/std/istream: Add cast to avoid -Wsign-compare.
	* include/std/stacktrace: Likewise.