If abs(__v) is smaller than one, the result will be of the
form 0.xxxxx. It is only if the magnitude is large that more digits
are needed before the decimal dot.

This uses frexp instead of log10 which should be less expensive
and have sufficient precision for the desired purpose.

It removes the problematic cases where log10 will be negative or not
fit in an int.

Signed-off-by: Paul Dreik <gccpatches@pauldreik.se>

libstdc++-v3/ChangeLog:

	PR libstdc++/110860
	* include/std/format (__formatter_fp::format): Use frexp instead
	of log10.

(cherry picked from commit 2d2b05f0)