[libc] add fast path to string to float conversion

Add the fast path first described by Clinger [1] with additions by Gay [2].
This speeds up conversion by about 10% by handling numbers with fewer digits
more efficiently.

[1] Clinger WD. How to Read Floating Point Numbers Accurately.
SIGPLAN Not 1990 Jun;25(6):92–101. https://doi.org/10.1145/93548.93557.
[2] Gay DM, Correctly rounded binary-decimal and decimal-binary conversions;
1990. AT&T Bell Laboratories Numerical Analysis Manuscript 90-10.

Reviewed By: sivachandra

Differential Revision: https://reviews.llvm.org/D112580