[libc] Refactor string to float return values

The internal implementation of the string to float function previously
used pointer arguments for returning several values. Additionally it
set errno in several unexpected places. Now all of that goes through
return structs. For readability I also moved the function away from raw
pointer arithmetic towards proper indexing. I also added support for
rounding modes.

Reviewed By: sivachandra

Differential Revision: https://reviews.llvm.org/D144597