libstdc++: Fix std::format("{}", 'c')

When I added a fast path for std::format("{}", x) in
r14-5587-g41a5ea4cab2c59 I forgot to handle char separately from other
integral types. That caused std::format("{}", 'c') to return "99"
instead of "c".

libstdc++-v3/ChangeLog:

	* include/std/format (__do_vformat_to): Handle char separately
	from other integral types.
	* testsuite/std/format/functions/format.cc: Check for expected
	output for char and bool arguments.
	* testsuite/std/format/string.cc: Check that 0 filling is
	rejected for character and string formats.