[libc] Fix printf %e and %g bugs

Fuzzing revealed bugs in the %e and %g conversions. Since these are very
similar, they are grouped together. Again, most of the bugs were related
to rounding. As an example, previously the code to check if the number
was truncated only worked for digits below the decimal point, due to it
being originally designed for %f. This patch adds a mechanism to check
the digits above the decimal point for both %e and %g.

Reviewed By: sivachandra, lntue

Differential Revision: https://reviews.llvm.org/D157536