libstdc++: Fix std::format alternate form for floating-point [PR108046]

libstdc++: Fix std::format alternate form for floating-point [PR108046]

A decimal point was being added to the end of the string for {:#.0}
because the __expc character was not being set, for the _Pres_none
presentation type, so __s.find(__expc) didn't the 'e' in "1e+01" and so
we created "1e+01." by appending the radix char to the end.

This can be fixed by ensuring that __expc='e' is set for the _Pres_none
case. I realized we can also set __expc='P' and __expc='E' when needed,
to save a call to std::toupper later.

For the {:#.0g} format, __expc='e' was being set and so the 'e' was
found in "1e+10" but then __z = __prec - __sigfigs would wraparound to
SIZE_MAX. That meant we would decide not to add a radix char because the
number of extra characters to insert would be 1+SIZE_MAX i.e. zero.

This can be fixed by using __z == 0 when __prec == 0.

libstdc++-v3/ChangeLog:

	PR libstdc++/108046
	* include/std/format (__formatter_fp::format): Ensure __expc is
	always set for all presentation types. Set __z correctly for
	zero precision.
	* testsuite/std/format/functions/format.cc: Check problem cases.