libstdc++: Fix std::format floating-point alternate forms [PR113512]

The logic for handling '#' forms was ... not good. The count of
significant figures just counted digits, instead of ignoring leading
zeros. And when moving the result from the stack buffer to a dynamic
string the exponent could get lost in some cases.

libstdc++-v3/ChangeLog:

	PR libstdc++/113512
	* include/std/format (__formatter_fp::format): Fix logic for
	alternate forms.
	* testsuite/std/format/functions/format.cc: Check buggy cases of
	alternate forms with g presentation type.