[libc] add printf decimal float conversion

This patch adds support for converting doubles to string in the %f/F
format specifier. It does not yet support long doubles outside of the
double range. This implementation is based on the work of Ulf Adams,
specifically the Ryu Printf algorithm.

See:
Ulf Adams. 2019. Ryū revisited: printf floating point conversion.
Proc. ACM Program. Lang. 3, OOPSLA, Article 169 (October 2019), 23 pages.
https://doi.org/10.1145/3360595

Differential Revision: https://reviews.llvm.org/D131023