[libc] Fix printf %f bugs

Fuzzing revealed several bugs in the %f float conversion. This patch
fixes them. Most of these bugs are related to rounding, such as
1.999...999 being rounded to 2.999...999 instead of 2.000...000 due to
rounding up not properly changing the nines to zeros. Additionally, much
of the rounding infrastructure has been refactored out so it can be
shared with the other conversions.

Reviewed By: sivachandra

Differential Revision: https://reviews.llvm.org/D157535