[libc++][format] Fixes floating-point formatting.

Formatting the alternate form for the general categories should keep the
trailing zeros. This was reported by @fsb4000 in D131336.

The default format uses general formatting but this should not keep the
trailing zeros so the default format is not passed to the formatter.

While testing I found an off by one error; finding the exponent character
`e` in 1e+03 will start at after the `1` so a size of `4` can contain an
exponent.

Reviewed By: fsb4000, ldionne, #libc

Differential Revision: https://reviews.llvm.org/D131417