[libc] add printf auto float conversion

[libc] add printf auto float conversion

This patch adds the final conversion to printf, %g. This is a floating
point conversion that selects automatically between the %e and %f
formats based on the precision requested and resulting exponent.
Additionally it trims trailing zeroes. With this done all that's left
for finishing printf is adding long double support to the decimal float
conversions.

Reviewed By: sivachandra

Differential Revision: https://reviews.llvm.org/D143006